Blender Git Loki

UI: Properties editor popover and outliner sync

This adds a popover to the properties editor. Currently the only setting
is for controlling outliner to properties syncing.

Because we cannot define a perfect heuristic to determine when
properties editors should change tabs based on outliner icon selection,
we need an option to enable or disable this behavior per properties
editor.

There are 3 options for controlling the syncing. Auto uses the heuristic
to only allow tab switching when a properties editor and outliner share
a border. On and off enable and disable syncing respectively.

Differential Revision: https://developer.blender.org/D9758