This patch basically adds all required parameters to play_rendered_anim() operator, allowing scripters playing back any rendered sequence or movie (according to the user preferences). Based on that work, I thought we can also provide a simple UI, which can be quite useful to review/playback the rendering without changing the current scene settings:
Personally, I never understood what the operator is doing under the hood and for that reason I always manually started an external player, I'm familiar with. However, I recently discovered the operator and I think it's actually a pretty nice feature if the user interaction would be better, so in addition to the popup I added some extra warnings and error messages as well, which should help users figuring out what's happening or what's might be wrong with the settings.
I'd appreciate any feedback.
Note: Successfully tested with all players except Framecycler (seems there is no standalone version anymore).