[MouseGestures] Implement "Rocker navigation"

Left+Right click - Forward
Right+Left click - Back

It is currently disabled by default and there is no GUI to enable it.
[MouseGestures] Configurable mouse gesture button

UI changes:
* the Settings dialog offers a combo box to select left and right
  mouse buttons, or disable the gestures completely
* changes are permanently saved
* OK button is added

Closes #386
Closes #1280

It is now also possible to enable/disable rocker navigation.

Closes #1234