Added ImGuiMouseCursor_None for usage by app/binding

imgui.h

@@ -664,6 +664,7 @@ enum ImGuiColorEditMode_
 // Enumeration for GetMouseCursor()
 enum ImGuiMouseCursor_
 {
+    ImGuiMouseCursor_None = -1,
     ImGuiMouseCursor_Arrow = 0,
     ImGuiMouseCursor_TextInput,         // When hovering over InputText, etc.
     ImGuiMouseCursor_Move,              // Unused