Fix playback buttons behavior between stills and clips

[zedsaeed]

Numerous controls and playback buttons, that are active and useable when playing video clips, remain active (but don't do anything) when switching over to stills. For example, controls such as FF and REW and others should be grayed out and not available when looking at a still. Either we should gray them out, or make them do something: for example, FF and REW buttons (when viewing stils) may be modified to quickly play the stills backwards or forwards, or go to the next still or the previous still...etc. Otherwise, just gray out the controls.