Fix the position of media controls in the playback with video

The media controls area (acorn controls) was being displayed at the
wrong place in the playback of a recording with video. Now it's always
displayed right below the presentation area, doesn't matter if the
recording has video or not.
Fix position of elements in the playback after swapping video/presentation

The media controls area (acorn controls) was placed in the wrong position
after swapping video and presentation in the recording playback (and was
still wrong if swapping them again).
Merge pull request #2 from daronco/playback-improvements

Fix the position of media controls in the playback with video