Draft: add a focus indcator to the waveform view

The focus indicator is a transparent outline drawn around the bar that represents the current time. It works correctly in RTL and LTR layouts, and is updated when the widget's focus changes whether or not the song is currently playing.

The one issue (that I know of 😄) left to figure out is what to do when at the beginning of the song. The current implementation won't draw any focus indicator in that case. Should we simply show the outline around the first empty bar instead of the last full one, as a special case? Or does this indicate a more fundamental problem with where I've decided to draw the focus ring? We could instead, for instance, draw it as a vertical line between the last full bar and the first empty one, which would allow consistent behavior regardless its position.

Closes #305.