Blender Git Loki

Add a new "shortening process" for UI string drawing, which keeps both right and left parts
of the drawstr and replaces the middle by '...'

Rationnal:
It's usually easier to figure out what a troncated string means if you have both its start and end parts.

Details:
It currently affects all UI items (text fields, buttons, labels...).
Only exceptions are buttons inside menus, text fields & co being edited, and numbuttons/sliders.
Note that some static texts (like panels' titles or items in outliner) do not use usual UI items,
and just draw the whole text, using OpenGL to clip it. Will make another patch to fix this.

Reviewers: campbellbarton, brecht, carter2422, #user_interface

Reviewed by: brecht, carter2422

CC: billrey

Differential Revision: https://developer.blender.org/D114