Shortcut schemes: support globally installed schemes.

This way an app can install schemes for users to choose from.

To make this less painful in case of kdevelop, I also added support for
loading all shortcuts from a single app file, rather than having to install
10 shortcut files. Saving into a single file requires changing the bool
saveToApplicationFile to true in the code though.

Finally, avoid saving empty xml docs (when saving one per component).