markers: Marker management shortcuts

Apart from adding markers and seeking between markers, there are no
current shortcuts available for managing markers.

Managing markers is therefore inefficient and time consuming.

To fix, marker management shortcuts for copying, cutting, pasting,
deleting, and editing the popover message were implemented.

GNOME/pitivi#2458
1 job for marker_mgmt in 20 minutes and 34 seconds (queued for 1 second)
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