Play Panel: persistent size and pos across sessions

*) Size and pos are read from QSettings when the widget is created
*) Size and pos are stored to QSettings when the widget is hidden/closed/destroyed
*) The member var Preferences::playPanelPos is removed, as the play panel pos is not configurable in preferences
Fixing rebase artifacts!
Merge pull request #326 from mgavioli/PlayPanel_persistent_size_pos

Play Panel: persistent size and pos across sessions