Tracking: support editing all selected tracks

This patch adds a "selected_movieclip_tracks" context member and enables
editing properties of multiple selected tracks via the usual Alt-click
editing (as well as the "Copy To Selected" operator). Both use
UI_context_copy_to_selected_list() to gather a list of other selected
items [which are now taken via said new context member]. Strictly
speaking, this could be done without the context member as well [just
gathering other selected tracks in UI_context_copy_to_selected_list()
without relying on a context member], but this might come in handy in
other places (e.g. Addons).

note: some could be desired for markers (e.g. editing pattern/search
areas of all selected track markers, but since this is burried in a
uiTemplate, this is a bit more work for another patch).

Differential Revision: https://developer.blender.org/D12923

doc/python_api/sphinx_doc_gen.py

@@ -1110,6 +1110,7 @@ context_type_map = {
     "selected_editable_sequences": ("Sequence", True),
     "selected_files": ("FileSelectEntry", True),
     "selected_nla_strips": ("NlaStrip", True),
+    "selected_movieclip_tracks": ("MovieTrackingTrack", True),
     "selected_nodes": ("Node", True),
     "selected_objects": ("Object", True),
     "selected_pose_bones": ("PoseBone", True),