xdgmenu: ported to GMenu 3

As now items use GIcons to store icon informations, the IconPool has
been changed to handle this case as well.
objects/baseobject: ensure that a value to be set is properly decoded.

This fixes the menu generation issue with some applications with special characters.
objects/icon: less strict type checking
objects/icon: use the default icon theme; use lookup_icon also for Gio.ThemedIcon and Gio.FileIcon.
config: use applications-other as xdgmenu-icon.
Merge branch 'devel' into debian-devel
debian/control: replace python-gmenu with gir1.2-gmenu-3.0.
1.30.0
Merge branch 'devel' into debian-devel
1.30.0-1