View Navigation: changes from review

Implemented:
* rename from fps to walk
* unifying walk and fly modes into VIEW3D_OT_navigate
* change up/down to Q/E keys
* expose gravity in the ui
* acceleration/deacceleration
* new shortcuts (MMB, ...)
* remove X/Z rolling
* teleport distance clamp to positive
* huge cleanup
* other small changes

Missing parts:
* walk->base_speed needs to be static or an operator settings
* Teleport should try to land in an offset to the hit point
* Speed could/should be relative to unit_settings.scale_length
* Testing/bug fixing
* Reverse mouse (I added a placeholder, but no functionality nor UI)

Not addressed:
* NDoF will not be supported for now
* We need a nice API to draw in the header while running the operator