Slic3r GUI not starting, error when using --gui (on linux)
Implements https://github.com/prusa3d/Slic3r/issues/407
Shortcuts: Movement in 3D Space
Assignment of the camera shortcuts is clear from the menu accelerators.
Implements https://github.com/prusa3d/Slic3r/issues/406
Shortcuts: Rotate +/- 45 degrees
l/r keys rotate the object
Also changed the accelerators for adding / removing duplicates from
Ctrl++/- to plain +/-, from Ctrl-Del to plain Del,
and added an 's' key accelerator for uniform scaling.
3b51f64411