This commit adds a toggle functionality to the `brush.asset_activate`
operator that makes it behave similarly to the `paint.brush_select`
parameter of the same name.
When the operator has this option enabled, using the operator or
pressing the relevant key will either:
* Activate the specified brush and store it if the current brush does
not match the specified brush
* Activate the previously stored brush if it exists.
This option is exposed in the keymaps and enabled by default for the
Sculpt Mask brush.
This allows, for example, users to press 'M' to switch to the mask brush
and then press 'M' again to switch back to their previously active
brush.
Partially addresses this RCS submission: [1]
[1] https://blender.community/c/rightclickselect/1VwZ/
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### Notes
This commit does not currently clear the `AssetWeakReference` when switching paint modes.
Pull Request: https://projects.blender.org/blender/blender/pulls/138845
Depending on internal details of how Blender is run, attempting to load
elements from the asset library may either execute as synchronous &
blocking or asynchronously.
When executing a script in background mode, prior to this commit,
operators that are dependent on the asset system will not execute
correctly due to the loading not being complete.
Busy-waiting for this by repeatedly calling the operator over and over
again in python does not resolve. To match behavior of other operators
when called from python scripts such as the quadriflow remesh, this
commit changes the `brush.asset_activate` operator and dependent code to
force a blocking call instead of optionally using the wmJob background
abstraction system.
Related to #117399
Pull Request: https://projects.blender.org/blender/blender/pulls/134203
