This was quite involved to get to work. Basic idea is to make
`bl_activate_operator` work for the pose library asset shelf, and
introducing a `bl_drag_operator` for blending poses.
- Make pose asset operators take an asset reference, which is how
`bl_activate_operator` usually gets the asset to operate on. This way
poses references can be assigned to a shortcut, identified by asset
library and relative asset path within the library. Falls back to
getting the asset from context.
- Trigger `bl_activate_operator` on every click, instead of only when
an un-active item becomes active. Needed so poses can be re-applied
as before.
- Fix button context not passed to the `bl_activate_operator` when
force-activating, e.g. on right-click events.
- Allow registering a `bl_drag_operator` in the asset shelf definition.
Executed when dragging an asset in the shelf.
- When dragging an asset, highlight it as active, without calling the
`bl_activate_operator`. This is important feedback to the user.
- Activate/select view items on click instead of drag, so dragging is
possible.
- Let pose applying operators handle the Ctrl key to apply poses
flipped. There's no simple way to attach such alternative behaviors
to `bl_activate_operator`/`bl_drag_operator`
- Remove keymap items that were there for the previous "hacky" solution
to apply & blend poses.
Pull Request: https://projects.blender.org/blender/blender/pulls/144023
