Changes to scrollbars so that they are always visible, but thin and
dim, and widen and become more visible as your mouse approaches.
See D6505 for details and examples.
Differential Revision: https://developer.blender.org/D6505
Reviewed by Campbell Barton
Moves all `interface_region*` files to C++ except for the tooptip region
which is slightly more complicated. Also move a few other files as well.
This helps to simplify and speed up code, especially through the use
of better C++ data structures. This change builds on all platforms on
the buildbot.
