provide additional state indicator/control when paused

Submitted by Jakub Steiner `@jimmac`

It's a common pattern to communicate a video is paused and allow easy play/resume. I think we should:

When video is paused, and OSD controls are shown, overlay a play button in the middle of the video. I'll provide a mockup and the assets.
Clicking anywhere on the video while OSD controls are shown, the video is paused or played back depending on its state.
It would possibly make sense to show the pause emblem over the playing video to communicate the behavior, but since we're using mouse movement to bring up the OSD, I suggest we don't shown the pause emblem for mouse input. Maybe for touch only.