Qmldesktop

Hi
As I worked for a while to provide new desktop UI based on QML there is my first version that I consider as good point to start next versions of implementation with more features currently present in gpodder GTK UI.
At the moment main things that are done:
- most windows prototypes as QML components
- adding podcasts (from podcast directory)
- downloading/playing episodes
- code was refactored to share desktop and mobile common qml code

Unfortunately there is no branch like qmldesktop in your tree, so there is pull request into the master.