ProfileManager dialog: improve handling profiles

- Differentiate between a profile being deleteable and writable:
  - Disable the Edit key for read-only profiles, i.e. a '.profile' that
    doesn't have write permissions for the user
  - Disable the Delete key for a '.profile' that is in a directory that
    isn't writable for the user (i.e. a .profile can be read-only, but
    still deleteable by the user)

- Change the model selection mode to single selection, that simplifies
  the code, besides it looks like editing more than one profile at the
  same time hasn't worked for a while, and there are no complaints AFAICS;
  remove the now redundant selectedProfiles() method.