preferences: create global settingsChanged signal

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>

core/subsurface-qt/divelistnotifier.h

@@ -83,6 +83,9 @@ signals:
 	// The core structures were completely reset. Repopulate all models.
 	void dataReset();
 
+	// The settings changed. Repopulate / rerender unit-dependent data, etc.
+	void settingsChanged();
+
 	// Note that there are no signals for trips being added and created
 	// because these events never happen without a dive being added, removed or moved.
 	// The dives are always sorted according to the dives_less_than() function of the core.