qt: ensure MRL are properly encoded and fix drag&drop with spaces

The default encoding QUrl::None maps to QUrl::PrettyDecoded, which is not entierely specified (documentation even mention it can change over different releases) and typically won't encode the spaces correctly.

When using drag&drop to play a media with characters that would need encoding, the system will provide the correct URL to the DropArea but Qt will encode it partially. Since we will encode it afterwards, we can just decode it completely and forward it as-is to the playlist controller.

Fixes dropping URL with spaces from a browser to VLC.