This removes mixes sequencer/preview display type.
Now preview supports editing operations it's impractical to support
displaying both at once as it complicates displaying actions in menus for e.g. which could apply
to both region types (select all, delete ... etc). See T92584 for details.
Existing files will set the view to show the sequence editor only.
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NOTE: this patch should be applied after {D13165}, to address the issue of insufficient vertical space for strip properties.