Fix memory leak when opening file browser

The name string of each file was duplicated but not freed. The new flag to
ensure the file-list frees the name wasn't set. Mistake in ca475479eb26.

Fix editors showing old data-block name after renaming in Outliner

E.g. the graph editor channels list would still show the old name of a material
that was renamed in the Outliner. Same for the Asset Browser when showing
data-blocks in the "Current File" repository.

The Outliner didn't send the notifier for some data-block types that editors
listen to for data-block name changes.

Fix Asset Browser showing old name after renaming data-block

The "Current File" asset library didn't get refreshed after the data-block name
changed. But rather than entirely refreshing the file list, or doing possibly
problematic partial refreshes, reference the data-block name directly, so a
simple redraw gets the new name displayed.

Addresses T83751

Asset Browser: Enable by default in alpha builds

The Asset Browser is still considered an experimental feature right now. But it
can be enabled by default for alpha builds, to get further testing and
feedback. It will hopefully be a non-experimental feature by the end of bcon1.
This is done in agreement with Dalai.

UI: Deselect other objects when dragging into 3D View

When dragging an object into the 3D View, e.g. from the Outliner or the Asset
Browser, other objects wouldn't get deselected. That differs from what other
drop operations do, which create a new object. They deselect other objects in a
lower-level function, which happens to not be called for just dropping objects.
So I guess this is an oversight.
Old behavior was also a bit annoying because users seem to expect this to
deselect.

Fix various issues with regions in Asset Browser

Fixes a number of glitches, e.g. the sidebar disappearing when selecting an
asset or wrong AZones (the little chevrons to indicate a hidden region).

There were a couple of issues:
* Execution region was created, but not used.
* If an execution region already existed when refreshing the area, it was
tagged as hidden, not removed.
* The sidebar was always set to be hidden on refreshes.
* When toggling from Asset Browser to File Browser as regular editor (i.e. not
opened temporary via Ctrl+O or such), the sidebar region wasn't removed.

Adresses T83644.

Fix T84684: Undo operators not working

Mistake in 2771dfd5632a. The commit left the new editors operator registration
in `ed_util_ops.c`, but removed the function call to it.
In other words, the ED-utils operators were not registered.

Fix T84684: Undo operators not working

Mistake in 2771dfd5632a. The commit left the new editors operator registration
in `ed_util_ops.c`, but removed the function call to it.
In other words, the ED-utils operators were not registered.

UI: Revert design changes to data-block selector for the 2.92 release

Partially reverts 2250b5cefee7.

Removing the user count and fake user count icons was controversial (which was
expected) and there are a few further changes needed, that won't make it in
time for the release, see D9946.
While there is a design to bring back the user count and fake user indicators,
a new design idea was proposed that the UI team wants to follow. This came too
late for the 2.92 release, the new design is targeted at the 2.93 release now.
Meanwhile, UI team decision was to simply revert the design changes.

The new design is being worked on in https://developer.blender.org/T84669.

Note that this commit does not revert some internal changes done in
2250b5cefee7. Namely the introduction of `ed_util_ops.c` and data-block
operators in there. These will still be needed in the new design.

Asset System: Disable Asset Browser as experimental feature

The Asset Browser will be disabled and not available for the 2.92 release. In
alpha/beta builds, there will be an "Asset Browser" option under Preferences >
Experimental, if the developer extras are enabled.
Note that this also disables related UI elements (e.g. "Mark Asset" buttons,
Preferences settings for asset libraries, etc.).

The code is still in master of course, development and testing will continue
there. But there simply needs to be too much polishing and fixing before the
2.92 release, plus there are some design decisions to be reevaluated.

Check the milestone 1 project to follow ongoing work:
https://developer.blender.org/project/view/124/