[Qt5] Add 5.3.0-beta

Adds the current beta, everything compiles nices and vlc-9999 and tomahawk work with it. As this is only a beta and scrolling with the mouse wheel seems to me broken, I do not want to remove the stable 5.2.1 release from the overlay.

Note: As the overlay is currently out of sync, johu's commit is shown in this pull-request, once the mirror is up-to-date again, it should vanish automatically.