Allow the user to select multiple recordings to play in sequence

[prl]

Add the ability to mark multiple recordings to play in sequence.

It would be useful to play the recording in the order that the user adds marks, rather than relying on the sort order. The recordings to play could be numbered rather than simple checkmarks to give feedback to the user about the play order.

Implementing #324: Add multi-file delete, move and copy to Movie Player could be done together with the implementation of this request because it requires similar UI elements.