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