adds basic implementation of materialwidgeteventhandler
no issue number because there is no (free) internet on airplanes sorry
Fix that with multiple canvases there is a comparison with a nullptr, ref #167
Closes #167, the second canvas works (currenty with the same, again loaded scene and the same painter till the render stages are included)
Merge branch 'materials/develop' of https://github.com/hpicgs/cgsee into materials/develop