Assets: Add Author field to asset metadata

This is information you'd typically want to be visible in the UI. It's optional
of course, so if not relevant, it can just remain unset.

Assets/IDs: Don't generate previews for object types with no real geometry

Object types like empties, cameras or lamps will just end up as empty preview
images. We can think about ways to visualize them still, but meanwhile, don't
create such an empty preview.

Differential Revision: https://developer.blender.org/D10334

Reviewed by: Bastien Montagne, Sybren Stüvel

UI: Refactor dropping support for the tree-view API

Introduces a dropping-controller API for the tree-view items,
`AbstractTreeViewItemDropController`. This reduces responsibilities of the main
tree-view item classes, which are already getting quite big. As I expect even
more functionality to be needed for it (e.g. drag support), it's better to
start introducing such controller types already.

Assets/UI: Improve asset library Preferences UI

* Open File Browser when pressing "Add Asset Library". This just makes sense,
since users have to select a directory for the asset library anyway.
* Move '+' icon back to the right side of the box. Then it is right under the
'x' icons for each indivdual library, which seems like the more natural
place.
* Correct tooltip for the "Add Asset Library" operator.
* Mark empty asset library name or paths field in red, to make clear that these
need to be set.

Asset Browser: Filter options for specific ID types

Adds a filter popup to the header that allows specifiying which data-block
types to show. The menu automatically reflects all supported ID types, so it
shows a checkbox for materials, worlds and actions currently by default, and
all ID types with the "Extended Asset Browser" experimental feature enabled.

UI: Correct icons of File Browser Link/Append ID filter settings

Wasn't using the same icons as usually used for these IDs, which can be
confusing. Corrected them to be consistent with other usages of these IDs.

UI: Show ID filter settings in Link/Append File Browser as icon & checkbox

The checkboxes integrate better with the surrounding layout and are not that
attention grabbing. To my knowledge the only reason not to use checkboxes was
so the icons could be displayed. But this does it just like the Outliner filter
settings: Show the icon before the checkbox.

Also widen the popover a bit to fit longer labels (didn't fit before this patch
even).

Assets: Allow specific data-block types to be enabled by default

Updates UI code so that we can enable the asset UI for specific data-block
types by default, i.e. irrespective of the "Extended Asset Browser"
experimental feature.
"Mark as Asset" and "Clear Asset" are always visible in the Outliner context
menu now, but are grayed out if not applicable and show a disabled hint in the
tooltip.

A known side-effect of this: The "Mark as Asset" and "Clear Asset" operators
are enabled for action data-blocks now, even though only pose actions created
through the Pose Libraries add-on are supported. If this is something worth
addressing is being discussed still.

Differential Revision: https://developer.blender.org/D12955

Reviewed by: Sybren Stüvel

Assets: Enable material and world assets by default

Both material and world assets should be ready to use as non-experimental
feature. They were not enabled by default yet because the work from the
previous commit was needed first.
Objects should follow soon.

Maniphest Task: https://developer.blender.org/T91752

Fix tooltip disabled hint not using correct context from the button

To display the "disabled hint" (text explaining why a button is disabled) in a
tooltip, it would run the operator poll callback, which could then set a poll
message. But the context for the poll check wasn't the one from the button, so
the poll may give a different result (and disabled hint) than the check of the
button itself did.
Make sure it uses the exact context from the button.