Map Alt + Tweaks to expected behaviours (rotate, zoom, move - LMB, RMB, MMB)

Increase stability of WALK_MOUSE_MOVEVERTICAL mode (MMB) by doing only one movement at a time

Take redraw and base speed into consideration for mouse move

This way speed modifiers (shift/alt) works and overall speed is more constant

Fix lookaround twist the view after awhile

Set default walk speed 10x faster

F (focus) shortcut to view_selected

Basic interaction file for unreal (LMB selection only at the moment)

Merge remote-tracking branch 'origin/master' into epic-navigation

Conflicts:
source/blender/blenlib/BLI_math_vector.h
source/blender/blenlib/intern/math_vector_inline.c
source/blender/editors/space_view3d/view3d_walk.c
source/blender/makesrna/intern/rna_wm.c

Fixup for MMB and LMB moves not working, introduced in last commit

Final adjustments to keymap

Cleanup pre-review

Navigation fully working

* Now you can start navigating with the arrows keys directly.

Keymap for arrows keys (and +, -)

Support for non-modal navigation (arrows)

Merge remote-tracking branch 'origin/master' into epic-navigation

All mouse modes working:

* RMB + Drag :
left/right | rotates horizontally
up/down | rotates vertically

* LMB + Drag :
left/right | rotates horizontally
up/down | move forward/backward

* MMB + Drag :
left/right | straft (mode sideways)
up/down | move up/down

Merge remote-tracking branch 'origin/master' into epic-navigation

View3D: Walk Navigation - initial support for Unreal Engine modes and keymap

Implemented:
* Mouse is reset back to its original location after the operation exits
* New Mouse Modes (see below)
* Trackpad support
* (keymap to enter and exit the walk navigation for the different mouse
modes)

Mouse Modes (wip):
* RMB + Drag : fully working
* LMB + Drag : rotation working, needs to move forward
* MMB + Drag : needs to move everywhere

(note, WASD work when using the mouse modes as well, just like in Unreal
Engine)

Not implemented yet:
* Direct (non-modal) keys: (arrow keys + PLUSPAD + MINUSPAD)

* G: game mode (equivalent of the 'show as render' option), GKEY may
be hardcoded in some places, so we may have to map it to something
else, but at least it will be nice to create an operator to toggle the
'show as render' option + the GLSL mode or something similar.

* Non-OSX support to UNTOUCH mouse/trackpad event

* Bugfix for mouse event still being called after UNTOUCH is detected
(this makes the operator to be called again if we swipe the mousepad
too fast and untouch quickly).

* F: Focus funcionality: Teleport to selected object

* All the other keys: I'm still not sure if we should try to map all the
Unreal Engine keyboard or only the basic navigation shortcuts

The main keys/functionality that need to be decided to be
implemented/included are:

* Z/C Change camera zoom (field of view)
* F11 to set fullscreen (the new editor fullscreen)
* Ctrl (or Cmd) + 1, 2, 3, ..., 0 to store the current view
* 1, 2, 3, ..., 0 to restore a saved view

All viewport controls in Unreal Engine:
https://docs.unrealengine.com/latest/INT/Engine/UI/LevelEditor/Viewports/ViewportControls/index.html

New math function: negate_v2_int()

New "Mouse/Trackpad" Untouch event for trackpads (OSX only for now)