Qt4 no longer supported

[Dirk Hohndel]

So 4.3 will be the last release that supports Qt4. As of a week ago or so
you'll need Qt5 to build Subsurface - and I am planning to remove the Qt4
specific special cases we had here and there prior to 4.4.

Unrelated to this I also made some changes to the way we create version
numbers. We now show the full git version (if built from git, that is) on
all platforms, both in the about screen and when starting Subsurface with
'--version'. And we have a consistent numbering scheme (I call them
'canonical version numbers') when calling the update check function. The
backend is now much smarter and notices if you are running a development
version and gives you more useful information.

I'd appreciate if people could test this a bit to make sure there aren't
any corner cases that I've missed. Whether you build from source or use
daily binaries, just click on Help->Check updates and verify that what you
see makes sense :-)

I'm also considering making the update check an opt-out function. I know
that some distros don't allow that, but I think I've made my point pretty
clear what I think of distros in general and those distros in particular.
They are free to drop Subsurface or hack it to turn this off by default.

We will continue to expand our set of 'official' binaries and continue to
steer people towards using those.

/D