Blender Git Loki

File Browser Arrow Keys Navigation

Adds support for selecting/deselecting files in File Browser using the
arrow keys. All directions (up, down, left, right) are possible.

When to Select, When to Deselect?
Standard behaviour is selecting, however if we move into a block of
already selected files (meaning 2+ files are selected) we start
deselecting

Possible Selection Methods
Simple selection (arrow-key): All other files are deselected
Expand selection (Shift+arrow key): Add to/remove from existing
selection
ill-Expand selection (Ctrl+Shift+arrow key): Add to/remove from existing
selection and fill everything in-between

From which file do we start navigating?
From each available selection method (Mouse-, Walk-, All-, Border
Select), we use the last selected file. If there's no selection at all
we use the first (down/right arrow) or last (up/left arrow) file.
(Ideally, the view would automatically be set to the new selection, but
this behaviour overlaps with an other patch I've been working on, so
prefer to do that separately)

(Also tweaks color for highlighted file for better feedback)

D1297, Review done by @campbellbarton, thx a lot :)