Bug#1123041: mplayer: port gmplayer to gtk3 and reintroduce mplayer-gui

[Lorenzo]

notes for myself in case the package comes back:

* make sure that localisation is enabled (if available) (#1036170)
* make sure to add icons and desktop file (maybe renamed to *-gui)
* check if drag and drop works (it doesn't with the latest gtk2 version)
* mplayer-skin-blue package is being removed too (#1122846): maybe a
  script to download skins is enough?