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

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