Implement Qt window system for onscreen windows (#21)
CR #335
CR #347:

* Cleaner way to figure out if window system need main thread event
  dispatch
* Cleaner destruction of qt::GLWidget using bound method
* Cleanups
Merge pull request #2 from eile/master

CR Eyescale/Equalizer#347
Merge pull request #347 from tribal-tec/qtreview

Implement Qt window system for onscreen windows