trying to stop calling into the MainWindow

[Dirk Hohndel]

We have created a monster.

Over couple of releases Tomaz kept pushing me to focus on this, but there
was always other stuff to work on. But given that we want to make progress
with subsurface-mobile this is the time to address this.

Right now our widgets are a tangled mess. Specifically, we keep calling
into the MainWindow for all kinds of purposes. But the MainWindow is part
of the desktop UI and needs to be removed from subsurface-mobile.

Tomaz has done quite a bit of work to get us to the point where this is
possible. In the process he managed to completely break subsurface-mobile
(which is OK - it's for a good cause). I continued this work today and
managed to untangle quite a bit of these dependencies, but a lot more work
is needed. I'm pushing this to master now because I want others to be able
to look at what I have done so far, comment on it, fix bugs I've
introduced, etc.

Subsurface, the desktop app, should continue to compile and work. I don't
think I broke this. Subsurface-mobile right now doesn't compile. It's
closer to compiling than it was this morning, but it will need a bunch
more untangling before that's possible.

/D