Improved playback/looping options

Prompted by the addition of Looping options in the 5.2 alpha release, I'm requesting more fleshed-out playback options.

In this concept:

Playback

Basic behavior

• Play Once: play once and stop
• Loop: loop forever
• Ping-Pong: bounce forward & backward forever

Start

Where to begin playing

• Range Start: timeline/preview range start
• Current Frame current frame/playhead location

End

Where to end playing

• Range End: timeline/preview range end
• Current Frame: current frame/playhead location

Return

How to behave once playback is over

• None: take no action
• Range Start: return to the start of the timeline/preview range
• Current Frame: restore the frame/playhead location

Variations that this concept would allow:

• play once from the start to the current frame: very good for straight-ahead animation
• play once from the current frame to the end, then return to the current frame
• ping-pong between current frame and end
• etc.