Fix: Crash when closing window while Outliner shows screens

Steps to reproduce were:
* From factory settings, change Outliner Display Mode to "Blender File"
* Open "Screens" item, make sure all listed screens are visible
* Open Preference window, close it
* Mouse hover the outliner -> crash

Fix is to force an Outliner tree rebuild when closing screens.

Fix T77830: Crash in VR session when opening material preview

Draw-manager mutex has to be set before activating OpenGL/GPU context.
Otherwise, parallel jobs (like preview rendering) may try to activate
the context from another thread.

Also: Use WM wrappers for activating/releasing OpenGL context, which
have an additional assert check.

Suggest to backport this for 2.83.1.

Fix T78112: VSE Sequencer/Preview crash after fullscreen

The region refresh callback wasn't called, which in the VSE ensures valid
region sizes and removes handlers of invisible regions.

Initial "Make Asset" operator

Accessible via the Outliner context menu (ID Data submenu).

The operator currently copies the data-block and creates empty asset data for
it. This will later contain all the meta-data to "give the data-block meaning".

Of course there are many TODOs and questions remaining. This is the first step
so I can work more on meta-data support.
I created some files and that I think make sense.

Merge branch 'asset-uuid--archived' into asset-engine--archived

Merge branch 'master' into asset-uuid--archived

Outliner/UI: Show library state icons for data-blocks in "Data API" mode

Previously it was not possible to tell appart a linked data-block from its
override copy. In fact you couldn't tell appart any data-blocks with the same
name (which is possible with linking).
Now we show the library state icon as we do for data-blocks in other display
modes.

Old/new comparison (note the overriden "GEO-head"):
{F8608835} {F8608836}

Fix T78455: Failed assert when opening a viewport panel

Mistake from 1fa40c9f8a810.

UI: Show library status icons in search menus, rather than letter prefix

Rather than a `L` (linked), `O` (overridden) or `M` (missing) prefix for the
name, show the existing library status icons. See D7999 for screenshots.

Note that when using preview icons, or if the search menu contains items with
own icons (e.g. brush icons), we still fallback to the prefix solution.

Zero or fake user is still indicated with a prefix.

Differential Revision: https://developer.blender.org/D7999

Reviewed by: Bastien Montagne, William Reynish

Fix compile error on Apple Clang

Needs to compile with `CMAKE_OSX_DEPLOYMENT_TARGET` of 10.14 at least to
have `std::optional<>::value()` available. So we have to increase the
minimum required compiler version, but that requires more changes. Just
fix the build error for now.