Add 'override' decl to fix compiler warnings, fix drag-and-drop.
This should shave off some 1,000+ warnings over the course of a build with GCC 9.1. This only addresses class declarations native to JuK, but Taglib and to a lesser extent Qt5 moc still have (or generate) code that can result in compiler warnings about suggested override declarations. In the process I noticed that an existing drag-and-drop support function (Playlist::decode) is no longer overriding Qt virtual functions, so it has been turned into an auxiliary function to fix existing drag-and-drop bugs from the KF5 port (tested by dropping files from Dolphin into a playlist view). Drag-and-drop from a playlist to another playlist (by dropping on the playlist name) still remains broken for now. CCBUG:392705 FIXED-IN:19.04.2