Don't edit the Fallback profile, instead create a new one

This is a first step in simplifying the code; since the Fallback profile
doesn't have a file on disk, it's basically a corner-case that we have to
babysit in various places in the code.

Now when the user tries to "Edit current profile", if it's the Fallback
profile, a new profile is created, with a unique name "Profile 1",
"Profile 2" ...etc. This is similar to using the "New" button in the
ProfileSettings dialog.

(cherry picked from commit 5ab84231)