songwidget: Handle checkbox interactions with selection-mode
There is a bidirectional binding between the coresong selected property and the checkbox active state. It keeps these properties synchronized. However, the selection change from the DiscBox is deactivated because a selection change would be propagated to the checkbox which would undo the selection. Solve this issue by adding a flag which keeps track of the clicks and prevents the same event from happening twice (one time in the checkbox and again in the SongWidget).