Polish drag/drop

[goto-bus-stop]

There's a few bits of polish that could be applied to media drag/drop to make it feel natural and like it always "just works".

• Hover the playlist manager with a history entry or a URL to open it
• Hover a playlist to select it
• Drop a playlist item in the same playlist to move it
• Drop a playlist item, search result, history entry, or URL into a playlist to add/"copy" it
• Drop a playlist item, search result, or URL onto a playlist to add it at the start
• Drop a history entry onto a playlist to add it at the end