Developed as part of the brush assets project, see #106303. No user visible
changes at this point.
Makes it possible to display asset shelves as popups. These popup asset shelves
use static storage for their settings, mainly to remember the active catalog
and filter string, while keeping them separate from the permanent asset shelf
region. Further, the popup can be displayed in any editor, making asset
selectors possible to add anywhere in the UI. When an asset is chosen, an
operator passed to the asset shelf as bl_activate_operator is called, with an
asset weak-reference to refer to the activated asset stored in the operator
properties.
Adds UILayout.template_asset_shelf_popover() to insert asset shelf popup
buttons, taking an asset shelf idname and some normal UI parameters.
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