Add new options to the Sub-Playlist widget:

• a checkbox called "Enable cycle based playback" with the help text "When cycle based playback is enabled only 1 Widget from this Sub-Playlist will be played each time the Layout is shown. The same Widget will be shown until the "Play count" is achieved."
• a number input called "Play count" with the help text "In cycle based playback, how many plays should each widget have before moving on".
• a checkbox called "Random Widget each cycle" with help text "When enabled a random Widget will be selected at the start of each cycle and shown until its play count has been met."

Player changes to obey cycle based playback and the play count:

• identifying Widgets which are sourced from a Sub-Playlist and treating them as a group
• tracking the play count of each one
• selecting the right widget to show when that widget group starts
• deciding what happens if the region loops
• obey the random widget option

This is closely related to #2437