Fast scrubbing (seeking) by dragging the time slider widget, with auto-pausing and auto-resuming

[nekohayo]

Most of the time, when I want to use a video player, I need to "scrub" through the video's visual frames to either find some particular scene, or to review a particular frame.

Currently (as of version 0.5.2), both in paused and playback mode, seeking only happens when you release the mouse button from a drag (or click) on the time slider.

Like other video players, it should

• Allow scrubbing through (async / without lag) by click-and-dragging the time slider, from any point in the slider's widget
  (not just by dragging the handle).
• Avoid interference from playback:
  • auto-pause during drag and auto-resume from the new position on drag release (if the video was already playing before scrubbing).
  • as long as the mousedown button state is held, do not allow playback to override the current position. If the mouse stops moving, the image should stay still.

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Bonus points if you can consistently show a popover with the playhead's timecode (and chapter name, if available) directly above the mouse cursor while hovering (and dragging) the time slider, like what Celluloid does. See issue #398.