Implemented select all for GDI

demo/gdi/nuklear_gdi.h

@@ -771,6 +771,13 @@ nk_gdi_handle_event(HWND wnd, UINT msg, WPARAM wparam, LPARAM lparam)
         case VK_PRIOR:
             nk_input_key(&gdi.ctx, NK_KEY_SCROLL_UP, down);
             return 1;
+                
+        case 'A':
+            if (ctrl) {
+                nk_input_key(&gdi.ctx, NK_KEY_TEXT_SELECT_ALL, down);
+                return 1;
+            }
+            break;
 
         case 'C':
             if (ctrl) {