Cleanup: Move interface_handlers.c to C++

Referenced by commit 6fdddae2b0, Fix T102804: Click & Drag on toggles no longer possible
Referenced by issue #102826, Regression: Click & Drag on toggles no longer possible
Referenced by issue #102804, Regression: Click & Drag on toggles no longer possible

source/blender/blenkernel/BKE_blender_undo.h

@@ -5,6 +5,8 @@
  * \ingroup bke
  */
 
+#include "BKE_undo_system.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -13,8 +15,6 @@ struct Main;
 struct MemFileUndoData;
 struct bContext;
 
-enum eUndoStepDir;
-
 #define BKE_UNDO_STR_MAX 64
 
 struct MemFileUndoData *BKE_memfile_undo_encode(struct Main *bmain,

source/blender/editors/interface/CMakeLists.txt

@@ -43,7 +43,7 @@ set(SRC
   interface_drag.cc
   interface_draw.cc
   interface_dropboxes.cc
-  interface_handlers.c
+  interface_handlers.cc
   interface_icons.cc
   interface_icons_event.cc
   interface_layout.cc

source/blender/editors/interface/interface_intern.h

@@ -1094,7 +1094,7 @@ const char *ui_textedit_undo(struct uiUndoStack_Text *undo_stack,
                              int direction,
                              int *r_cursor_index);
 
-/* interface_handlers.c */
+/* interface_handlers.cc */
 
 extern void ui_handle_afterfunc_add_operator(struct wmOperatorType *ot,
                                              wmOperatorCallContext opcontext);