Implement new Notifications KCM

It's only UI with no actual notification reading/writing right now but the design
is pretty much done.

It has settings for Do Not Disturb mode (time-based, activity-based, etc) and moves some of the
settings that are currently spread out all over Plasma into a central place (e.g. task manager
badges and progress as well as popup position settings, as the new notification system will
only have one popup group in total, not one per applet)

There's also support for third-party apps (just server-side popup on/off, no elaborate notifyrc stuff)
and various other bits and pieces.