Shortcuts: Add a way for applications to add shortcuts when installed

[jackburton]

There should be a way for applications to add new shortcuts to the system. We could implement this in a way similar to how Expander works: An application package could put a file into shortcuts's (or input_server's) folder, with the definition of the new shortcuts.

A problem I can see with this approach, though, is that there is no way to check for shortcuts conflicts.