Blender Git Loki

UI: Fix odd behavior in region sizing, simplify code

* When resizing sidebars, don't collapse when the region becomes too big but
instead clamp the region size to the available space.
* Fix clicking the tab to expand sidebars no working if the sidebar is too
wide to fit. Instead make it less wide so it does fit.
* Fix incorrect limit on tool properties region height, for example in the
file browser.

Differential Revision: https://developer.blender.org/D4611