GtkImageMenuItem has been deprecated, but we will want to continue
to use image menu items at least in Main Menu and Menu Bar applets.

Added files are copied from GTK+ 3.8.9 version and then modified:
- GtkImageMenuItem renamed to PanelImageMenuItem.
- Removed deprecated parts - GtkStock, GtkActivatable.
- Changed license from LGPL to GPL. This should be allowed. At
  least that is how I understand it. See third paragraph in LGPL
  license.

We already had panel_image_menu_new function. This commit renames
function name to panel_image_new_new2 to not conflict with newly
added files.