Remove the checks for region size in operator_markers_region_active as
that interferes too much. The only thing really interfering with
playhead movement when the area gets too narrow is MARKER_OT_select. So
this PR adds a new poll function for that in particular. That allows a
lot of simplification.
Pull Request: https://projects.blender.org/blender/blender/pulls/137399
d6d6a7e36f