Initial commit of Qml Desktop GUI
Most windows created. Need icons and better layout.
Gui shows at startup. Menubar introduced.

run with --qml_desktop to get QML for desktop
Translation function introduced. Renamed all files.
Added new components, names changed, GUI updated.
Controller code reorganization. GUI updated. Playing and downloading works.
Podcast directory works. GUI improvements, code reorganization.

Episode list implemented as TableView instead of ListView
Improvements to the extension system

- Add category metadata in every extension
- Show this category in the extension list gui
- Add "mandatory_in" and "disable-in" configuration for an extension
- Add Ubuntu unity check to enable/disable unity specific extensions
- Move "gpodder.win32" and "gpodder.osx" setting to the "gpodder.ui" namespace to be able to use it in the extensions category settings
- Only show metadata information in the right-click dialog of an extension
Dont warn about system files in windows (bug 1712)
gpodder.util: Use gpodder.ui.win32 for new code
Add disable_in=win32 to a few extensions (bug 1729)
QML UI: Cleanup and simplify episode list
Initial QML Performance Changes
Some more tweaks to the QML UI
Episode loading animation
QML UI: Avoid crashes with zero refcounts
QML UI: Multi-Episodes Sheet should fill screen
QML UI: Only show Episode Actions with listview
QML UI: Show download feedback immediately
QML UI: Show subscribe list in Sheet