Several menus can be triggered either as popups on their own when right-clicking
the dockable or on items, or as a submenu in the docks main menu.
In the former case, we want the menu items to be at the root level, whereas in
the latter case, we want them inside a properly named submenu.
To handle this, I wrap these in a properly labelled <submenu> in the .ui files,
and I add a boolean flag to gimp_menu_shell_fill() which will drop the top
submenu if and only if this is the only top item. I use this flag only when
using these menus as popup menu on their own.
Also fixing a few memory leaks around these pieces of code.
Anders Jonsson