VS-1680: Arrows Inward [F4 (---> <---]

Tags: vs-1680
---> <--- is assigned to the [F4] function key whenever the Playlist is
displayed.

[F4 ( ---> <--- )]: This reduces (in four stages) the display of the Playlist
in the time area. When the waveform is displayed, this reduces the display of
the waveform in the time area.