Give "Previous marker" function a larger frame window to allow rapid presses during playback

Currently during VSE playback, you can only reliably traverse backwards through markers if playback is off. If playback is in progress, you can't press the hotkey fast enough to move back through.

Currently it may be a 0-1 frame window, but this is basically too fast for human reflexes to control (seems likely, given holding the key down until it repeats does move back through).

This isn't a huge issue (one can pause then move backwards through markers), but it's a bit unconventional compared to most standard playback behaviour.

Hopefully the code changes to modify this behaviour for the "previous marker" function would be fairly minor.