Making testing easier in IPFire 2

This patchset makes testing easier in IPFire 2.

Since we are now trying to use the nightly builds more, pakfire is now automatically configured to use the packages from the correct tree.

That means that testing add-ons will be easier without any manual changes of any configuration files. Yay.

Additionally, there is now a dropdown on the pakfire page where users can change from one tree to another one which makes testing core updates easier, too.

Please review my changes and let me know if I messed anything up.

Best, -Michael