Misc improvements: port to Python3, proper install, panning with new mouse-button layout, more test files

[Spiritdude]

Excuse these manifold improvements as a single PR (best view [Files changed] and not the individual commits), my work style doesn't align well with git.

Following changes I made:

• new version numbering with three parts (0.5.x)
• port to Python3
• adding Makefile with make install
• setup.py with proper version of pyglet (essential)
• new color scheme with white background
• panning implemented, therefore new mouse-button layout to match OpenSCAD
• detection of wall, infill, support material and dedicated colors, based on G-code comments
• non-planar layers support (layers are recognized with ';LAYER:(\w+)') not Z changes where possible
• more G-code tests with original STL files: Slic3r, PrusaSlicer, Cura, etc
• adding LICENSE (BSD)