[wip] Status notifier support

[mtwebster]

Adds support for org.freedesktopkde.StatusNotifier spec.

A small support program will advertise sn support, sn (and appindicator and ayatana) users will connect, this program will spawn an XAppStatusIcon to add to an applet.

Ref:
linuxmint/cinnamon#9176

Behavior -

Watcher is dbus-launched by any applet that uses XAppStatusIconMonitor. This is a singleton and lasts only as long as any monitors remain running. For instance, if you remove cinnamon's xapp status applet, the watcher will terminate.

Watcher allows any StatusNotifier-types to register, and creates an XAppStatusIcon for each, which are then shown in the applet.

The watcher acts as a StatusNotifierWatcher, and the xapp is essentially a StatusNotifierHost (https://www.freedesktop.org/wiki/Specifications/StatusNotifierItem/)

Impacts:

• Should allow us to remove existing indicator support from Cinnamon
• Will allow non-patched appindicator programs to have their icons displayed here..
• If we ditch the patched libappindicator, we'll (probably) lose the ability to check whether or not a program has assigned a 'secondary activation' action - this is ordinarily meant for the middle mouse button, but we generally prefer a left-click performing any activation, rather than showing a menu with both left and right. So, the consequence is, if a program has no secondary activation, left-click will do nothing. Only right click will bring up the menu. If it does have activation, left-click will activate.