[3.3.x] Iterate on PlaybackSpeedView

This renames NewPlaybackSpeedView to PlaybackSpeedView as it is the only playback speed view currently present.

Additionally, this manipulates the xib file to remove unneeded (stack) views and to set a fixed view height. With that fixed height, the panel is much smaller and no longer hides the subtitles when trying to set their delay. Additionally, this removes the dark background when the panel is shown to be actually able to see the video when manipulating delays.

Backport of !734 (merged)