Don't consume all 'C'/Insert key presses in wxGrid

Skip the key events other than Ctrl-C/Ins which are used for copying
grid contents to the clipboard, notably Alt-C which should still be
usable for opening the menus.

This fixes a problems introduced in 67c1c412c6 (Implement support for
copying wxGrid cells to clipboard, 2020-04-26), see #13562.

src/generic/grid.cpp

@@ -5955,8 +5955,9 @@ void wxGrid::OnKeyDown( wxKeyEvent& event )
                     }
 
                     wxTheClipboard->SetData(new wxTextDataObject(buf));
+                    break;
                 }
-                break;
+                wxFALLTHROUGH;
 #endif // wxUSE_CLIPBOARD
 
             default: