[feature] Action to set a feature as the current atlas feature - sponsored by SIGE Edit - version 2

This request supercedes https://github.com/qgis/QGIS/pull/1061 . I've now created a QgsMapLayerAction and QgsMapLayerActionRegistry as suggested by @wonder-sk. This means there's now no changes to QgsVectorLayer. There's a few small cleanups I still need to do with the new "Set as atlas feature" action (namely, make the action text show the composition it applies to), but I'd like to get these major changes reviewed and committed first.