InputText: Moved code for checking clipboard shortcut so we can expand on it. (#1541)

TODO.txt

@@ -64,6 +64,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - input text: add discard flag (e.g. ImGuiInputTextFlags_DiscardActiveBuffer) or make it easier to clear active focus for text replacement during edition (#725)
  - input text: display bug when clicking a drag/slider after an input text in a different window has all-selected text (order dependant). actually a very old bug but no one appears to have noticed it.
  - input text multi-line: don't directly call AddText() which does an unnecessary vertex reserve for character count prior to clipping. and/or more line-based clipping to AddText(). and/or reorganize TextUnformatted/RenderText for more efficiency for large text (e.g TextUnformatted could clip and log separately, etc).
+ - input text multi-line: support for cut/paste without selection (cut/paste the current line)
  - input text multi-line: line numbers? status bar? (follow up on #200)
  - input text multi-line: behave better when user changes input buffer while editing is active (even though it is illegal behavior). namely, the change of buffer can create a scrollbar glitch (#725)
  - input text multi-line: better horizontal scrolling support (#383, #1224)

imgui.cpp

@@ -8610,6 +8610,10 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
         const bool is_wordmove_key_down = io.OptMacOSXBehaviors ? io.KeyAlt : io.KeyCtrl;                     // OS X style: Text editing cursor movement using Alt instead of Ctrl
         const bool is_startend_key_down = io.OptMacOSXBehaviors && io.KeySuper && !io.KeyCtrl && !io.KeyAlt;  // OS X style: Line/Text Start and End using Cmd+Arrows instead of Home/End
 
+        const bool is_paste = (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_V)) && is_editable;
+        const bool is_cut = (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_X)) && is_editable && !is_password && (!is_multiline || edit_state.HasSelection());
+        const bool is_copy = (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_C)) && !is_password && (!is_multiline || edit_state.HasSelection());
+
         if (IsKeyPressedMap(ImGuiKey_LeftArrow))                        { edit_state.OnKeyPressed((is_startend_key_down ? STB_TEXTEDIT_K_LINESTART : is_wordmove_key_down ? STB_TEXTEDIT_K_WORDLEFT : STB_TEXTEDIT_K_LEFT) | k_mask); }
         else if (IsKeyPressedMap(ImGuiKey_RightArrow))                  { edit_state.OnKeyPressed((is_startend_key_down ? STB_TEXTEDIT_K_LINEEND : is_wordmove_key_down ? STB_TEXTEDIT_K_WORDRIGHT : STB_TEXTEDIT_K_RIGHT) | k_mask); }
         else if (IsKeyPressedMap(ImGuiKey_UpArrow) && is_multiline)     { if (io.KeyCtrl) SetWindowScrollY(draw_window, ImMax(draw_window->Scroll.y - g.FontSize, 0.0f)); else edit_state.OnKeyPressed((is_startend_key_down ? STB_TEXTEDIT_K_TEXTSTART : STB_TEXTEDIT_K_UP) | k_mask); }
@@ -8650,13 +8654,9 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
         else if (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_Z) && is_editable && is_undoable)      { edit_state.OnKeyPressed(STB_TEXTEDIT_K_UNDO); edit_state.ClearSelection(); }
         else if (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_Y) && is_editable && is_undoable)      { edit_state.OnKeyPressed(STB_TEXTEDIT_K_REDO); edit_state.ClearSelection(); }
         else if (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_A))                                    { edit_state.SelectAll(); edit_state.CursorFollow = true; }
-        else if (is_shortcut_key_only && !is_password && ((IsKeyPressedMap(ImGuiKey_X) && is_editable) || IsKeyPressedMap(ImGuiKey_C)) && (!is_multiline || edit_state.HasSelection()))
+        else if (is_cut || is_copy)
         {
             // Cut, Copy
-            const bool cut = IsKeyPressedMap(ImGuiKey_X);
-            if (cut && !edit_state.HasSelection())
-                edit_state.SelectAll();
-
             if (io.SetClipboardTextFn)
             {
                 const int ib = edit_state.HasSelection() ? ImMin(edit_state.StbState.select_start, edit_state.StbState.select_end) : 0;
@@ -8666,13 +8666,15 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
                 SetClipboardText(edit_state.TempTextBuffer.Data);
             }
 
-            if (cut)
+            if (is_cut)
             {
+                if (!edit_state.HasSelection())
+                    edit_state.SelectAll();
                 edit_state.CursorFollow = true;
                 stb_textedit_cut(&edit_state, &edit_state.StbState);
             }
         }
-        else if (is_shortcut_key_only && IsKeyPressedMap(ImGuiKey_V) && is_editable)
+        else if (is_paste)
         {
             // Paste
             if (const char* clipboard = GetClipboardText())