Support notification manipulation (enable / disable) via GNOME Control Center.

[JoshStrobl]

This pull request implements support for silencing / (enabling / disabling) notifications via GNOME Control Center, which manipulations applications via /org/gnome/desktop/notifications/application/APPNAME. By default, no enable key is set for these applications, therefore they are enabled and we only apply the setting if the bool key exists.

Note that this doesn't apply to snap applications such as Spotify (which seem to get set by GNOME Control Center with names like spotify-spotify instead of just spotify), but figured it'd be better than nothing.

This has been tested against:

• Peek
• Rhythmbox
• Spotify (Snap)
• notify-send: To validate that there shouldn't be critical issues if it fails to get a schema.

Further notes:

• This should be merged in before Implement broader Raven DND support and tweak Clear Notifications. #1451 and I will happily rebase Implement broader Raven DND support and tweak Clear Notifications. #1451 on the changes in this commit (namely around the fact that mute_control was changed to dnd_enabled in that PR).
• This should resolve Evolution notifications #926 and [Feature Request] Exclude Notifications From Notifications List #1286.

[ikeydoherty]

GRMA :D

[fn-ix]

I don't believe this solves #1286: it only allows enabling and disabling of notifications, not of their appearing in the notifications drawer separately from the pop-up, besides the option to have no pop-up but still have an entry in the drawer (but not vice versa, which was the point of #1286).

I.e. I'd like an e-mail notification to end up in the drawer and highlight the bell, whilst still getting pop-ups from a music player about the new song or a "ready to unmount" message from the file manager, but not have them populate the drawer, making it useless as an indicator of rarer and more important events and forcing me to regularly empty it.