This is a minimal, information-only view currently, listing by default
all the override data-blocks, with their user-edited override
properties.
System-generated overrides (like the overrides of pointers to other
override data-blocks) can be shown through a filter option.
Finally, potential info or warning messages from (auto-)resync propcess
are also shown, as an icon + tooltip next to the affected items.
Part of D10855.
No functional changes. Code is ported to C++ with additional cleanups to
the logic and variable names.
Differential Revision: https://developer.blender.org/D9741
See https://developer.blender.org/D9499.
"View" leads to weird names like `TreeViewViewLayer` and after all they are
specific to what we call a "display mode", so "display" is more appropriate.
Also add, update and correct comments.
