Port to GAction and libpanel-applet 3.14.

References:
https://wiki.gnome.org/HowDoI/GAction
https://wiki.gnome.org/HowDoI/ApplicationMenu
https://mail.gnome.org/archives/gnome-announce-list/2014-October/msg00036.html
Properly organize items into submenus
Action names should be lowercase
Fix mode actions
Fix toggle actions