sync with Unstable

Standing merge request to synchronize changes from unstable branch into master branch:

changes:

• reverted changed include order, as it broke windows compilation
• separated parts of QtOpenGL functions to new .cc files and used more forward declarations in Header files, as preparation to remove glew.

if the OpenFlipper-Free mr passes, please also merge this one. https://graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/merge_requests/305