gapplication: Add an org.gtk.Debugging interface

Submitted by Philip Withnall @pwithnall

Link to original bug (#769841)

Description

The idea is that each application (or service) should provide a uniform external interface for enabling and disabling debug output — essentially a runtime equivalent of G_MESSAGES_DEBUG. This could be used by systemd and GNOME Builder to control debug output of processes without having to restart them.

Currently, the interface suggestion is:

  <node>
    <interface name='org.gtk.Debugging'>
      <property name='EnableDebug' type='b' access='readwrite'/>
    </interface>
  </node>

I hope that eventually the interface could be renamed to org.freedesktop.Debugging once we're sure it's stable.