So far, the PreferencesDialog emitted a settingsChanged signal.
This meant that models that listened to that signal had to
conditionally compile out the code for mobile or the connection
had to be made in MainWindow.
Instead, introduce a global signal that does this and move
the connects to the listeners to remove inter-dependencies.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
