Add the ability to read cloud providers from XDG desktop files

Closes #7 (closed). The one change I made relative to the original idea is to use org.freedesktop.CloudProviders as the Implements= value, since using org.freedesktop.CloudProviders.Provider might imply that the object should only implement that interface, even though it can implement both. The XDG spec itself just says the names just have to be valid D-Bus names and are left up to interpretation.

I also had it automatically grab the bus name from the .desktop file name if a custom name is not given, which means that this will play really nicely with e.g. D-Bus activation!

(I'm also not quite sure why so many commits that are already on master are appearing here, I guess maybe it's just a UI bug like happens with GitHub?) Okay fixed.