Add submenus for custom actions (Issue #184)

Attention: Before testing this MR, best backup your uca.xml to prevent trouble !

I tried to get the order inside the submenu correct, but that resulted in having a new thunarx API method "thunarx_menu_prepend_item" ... I supose not worth do bump the thunarx API version just because of that.

Note that you need to install the patched thunar to try this patch. Does not run directly from the source folder.

Suggestions and critic are very welcome !