Blender Git Loki

UI: Redesigned data-block selectors

The previous design is rather old and has a couple of problems:

* Scalability: The current solution of adding little icon buttons next to the
data-block name field doesn't scale well. It only works if there's a small
number of operations. We need to be able to place more items there for better
data-block management. Especially with the introduction of library overrides.
* Discoverability: It's not obvious what some of the icons do. They appear and
disappear, but it's not obvious why some are available at times and others
not.
* Unclear Status: Currently their library status (linked, indirectly linked,
broken link, library override) isn't really clear.
* Unusual behavior: Some of the icon buttons allow Shift or Ctrl clicking to
invoke alternative behaviors. This is not a usual pattern in Blender.

This patch does the following changes:

* Adds a menu to the right of the name button to access all kinds of operations
(create, delete, unlink, user management, library overrides, etc).
* Make good use of the "disabled hint" for tooltips, to explain why buttons are
disabled. The UI team wants to establish this as a good practise.
* Use superimposed icons for duplicate and unlink, rather than extra buttons
(uses less space, looks less distracting and is a nice + consistent design
language).
* Remove fake user and user count button, they are available from the menu now.
* Support tooltips for superimposed icons (committed mouse hover feedback to
master already).
* Slightly increase size of the name button - it was already a bit small
before, and the move from real buttons to superimposed icons reduces usable
space for the name itself.
* More clearly differentiate between duplicate and creating a new data-block.
The latter is only available in the menu.
* Display library status icon on the left (linked, missing library, overridden,
asset)
* Disables "Make Single User" button - in review we weren't sure if there are
good use-cases for it, so better to see if we can remove it.

Note that I do expect some aspects of this design to change still. I think some
changes are problematic, but others disagreed. I will open a feedback thread on
devtalk to see what others think.

Differential Revision: https://developer.blender.org/D8554

Reviewed by: Bastien Montagne

Design discussed and agreed on with the UI team, also see T79959.