Blender Git Loki

UI: Internal support for custom UI list item drag & activate operators

For pose libraries, we need to be able to apply a pose whenever
activating (clicking) an item in the Pose Library asset view and blend
it by dragging (press & move). And since we want to allow Python scripts
to define what happens at least when activating an asset (so they can
define for example a custom "Apply" operator for preset assets), it
makes sense to just let them pass an operator name to the asset view
template. The template will be introduced in a following commit.