Fixes#9883
There were menus that put their entries
directly in the tab menu rather than in a submenu.
This moves them to submenus to bring back GIMP 2.10 behavior.
Stray entries added by scripts are also moved to the right
submenu.
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.