Cleanup: remove unused button function

rB05f900e3466b45a19e13bea6dd641e4f7b8b46e9 removed unused button functions,
but since that commit the `uiDefIconTextButBit()` static function sits
unused as well. It's now been removed.

Avoid exception when no weight paint settings exist

Just an extra check for `None` before accessing its properties.

Cleanup: remove BKE_animdata_driver_path_hack

The `BKE_animdata_driver_path_hack()` function has had almost no effect
since rB51b796ff1528, and basically boils down to:

```
return base_path ? base_path : RNA_path_from_ID_to_property(ptr, prop);
```

Since `base_path` was `NULL` in the majority of cases, it's just been
replaced by a direct call to `RNA_path_from_ID_to_property()`. The
conditional now just appears in one remaining case.

This relates to T91387.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D13646

Fix T93960: Asset Catalogs I/O fails with unicode file paths on Windows

On Windows, encode file paths as UTF-16 before trying to open the file
for reading/writing.

This introduces a new class `blender::fstream`, which wraps
`std::fstream` and provides this UTF-16 encoding. This class should also
be used in other areas, like the Alembic importer/exporter.

Manifest Task: T93960

Reviewed By: JacquesLucke

Differential Revision: https://developer.blender.org/D13633

Assets: log message when catalog definitions cannot be loaded

Log a message (via `CLOG`) when asset catalog definitions cannot be
loaded.

Reviewed by @jacqueslucke in D13633

Review comments from Jacques

Tests: initialise BKE callbacks before loading blend file

Initialise the BKE callback system in
`BlendfileLoadingBaseTest::SetUpTestCase()`. This allows certain tests
to run in debug mode (when `BLI_assert` is enabled).

Fix T94116: Drivers can have multiple variables with same name

The RNA setter now ensures that driver variables are uniquely named
(within the scope of the driver).

Versioning code has been added to ensure this uniqueness. The last
variable with the non-unique name retains the original name; this
ensures that the driver will still evaluate to the same value as before
this fix.

This also introduces a new blenlib function `BLI_listbase_from_link()`,
which can be used to find the entire list from any item within the list.

Manifest Task: T94116

Reviewed By: mont29, JacquesLucke

Maniphest Tasks: T94116

Differential Revision: https://developer.blender.org/D13594

Cleanup: correct docstring for `driver_variable_name_validate`

No functional changes.

Animation: send notifier when keyframe is inserted

`<some_id>.keyframe_insert()` now sends a notifier that animation data
was changed, so that animation-related editors can properly refresh.

Since this function is quite high-level (if necessary it creates the
Action and FCurves), I thought this would be a suitable location for the
notifier. If high keyframing speed is required, it is still recommended
to use `FCurveKeyframePoints.insert(options={'FAST'})` instead.