View Navigation: Walk and Fly modes

This is a replacement for the dynamic fly mode.
It behaves as the first person navigation system available
in most 3d world  games nowadays.

Shortcuts:
----------
Shift+F             - Enable navigation mode (Walk/New Fly)
Ctrl+Shift+F     - Old Fly mode
WASD (hold)  - Move forward/backward and straft left/right
QE (hold)    - Move up and down
F            - Fly mode
R            - Walk Mode
Tab          - Alternate between Walk and Fly modes
Shift (hold) - Speed up movement
Alt (hold)   - Slow down movement
Space        - Jump (teleport when in fly mode)
T            - Teleport

User Preferences Options:
-------------------------
Navigation Mode   - fly/walk navigation systems
Mouse Sensitivity - sensitivity factor to mouse influence to look around
Teleport Duration - how long the duration last
Camera Height     - camera height to use in walk mode
Jump Height       - maximum jump speed in m/s
Move Speed        - base move speed in m/s
Boost Factor      - multiplication factor when running or going slow (1/boost)

Development Notes:
------------------
* The initial code was based on view3d_fly.c.
* The NDoF code was not touched, so it most likely is not working.
* I removed the deacelerate/accelerate code.

Pending Issues:
---------------
* The axis roll correction is disabled (it was affecting the move around code.
* Draw in the UI the shortcut options, and current values (e.g., Mode: Fly/Walk)
* We can rename Fly to Free, so Walk/Free Navigation is provided by this code, and Fly can use the old code.
* They would need to be integrated as one operator somehow (the code can be merged, but to evaluate whether it's worthy).