main: support sub-menus in guitool actions

main: support sub-menus in guitool actions

Teach the Actions menu to treat slashes ('/') in guitool action names as
submenu delimeters for grouping related actions inside submenus.

Create menus underneath the top-level Actions menu when the action name
contains slash-delimited menus. Nested submenus are supported by using
multiple slash-delimited "paths" in the name.

For example, an action called 'Tools/Git/Cola' would create a 'Tools'
menu inside the top-level 'Actions' menu, a 'Git' submenu inside the
'Tools' menu and an action called 'Cola'.

Closes #1150
Suggested-by: Sean Allred <code@seanallred.com>
Signed-off-by: David Aguilar <davvid@gmail.com>