SourceCode Merge Request #12: Enhanced keyboard modificator handling on the 3d canvas (merged)

This patch removes modificator key (Ctrl, Shift, Alt) handling from dxf2gcode.py and moves them where they should be - into the Canvas classes.

• Left-click with Alt pressed will restore rotation to (0,0,0)
• Modificators won't "stick" when you press a modificator and move the mouse out of the window or, say, press Ctrl+O
• Modificator handling is done in CanvasBase, thus leaving space for unification with Canvas2d in future