* gui: camera navigation style option
Add camera navigation style config option as a preparation for
introduction of touchpad style navigation
* gui: touchpa-friently 3d navigation
Implement FreeCAD-inspired touchpad-friendly camera operation mode:
- Shift+move: panning
- Alt+move: rotation
Set "Camera mode" to "Touchpad" in settings menu to activate this mode.
* cache current navigation style in class member variable
Cache current navigation style on canvas focus, do not
request settings value each mouse event.
* force focuse 3d preview on preferences window close
Explicitly force focus 3D preview pane on closing preferences window.
This allows preferences be updated only once without checking current
value each input event.
Another benefit is to have 3D view in focus is that user could
immediatelly use 3d-view shortcuts like Ctrl+<1..7> without clicking to
the 3D view first.
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