For some time, the desktop file specification has supported "additional
application actions".  This is intended to allow for additional methods
of starting an app, such as a mail client having a "Compose New Message"
action that brings up the compose window instead of the folder list.

This patch adds support for this with a relatively minimal API.

In the case that the application is a GApplication and DBusActivatable,
desktop actions are translated into GActions that have been added to the
application with g_action_map_add_action().  This more or less closes
the loop on being able to activate an application with an action
invocation (instead of 'activate').

https://bugzilla.gnome.org/show_bug.cgi?id=664444