Tracks are selected to match a given length.

A combo box is displayed to select the implemented random choice strategies (tracks of a random album, tracks of a random album, or random tracks for half an hour).

To start with, no need to offer multiple lengths: Users can make two selections of half an hour to populate a one hour tracklist. Also no need for a list of selected tracks right now, since users can view those tracks and remove somes from the tracklist view.

Original issue: #106