Code cleanup: implement window title update via signal

This seems quite convoluted to me but I can't seem to make a more straight
forward implementation work. The idea is that core code should never
directly call into the UI.

So instead the core code (this is C code) calls a helper function. That
helper function calls a member function of a class which in return emits a
signal. The UI code connects to that signal and acts accordingly when it
is received.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qthelper.cpp

@@ -1073,11 +1073,6 @@ extern "C" bool canReachCloudServer()
 	return CheckCloudConnection::checkServer();
 }
 
-extern "C" void updateWindowTitle()
-{
-	MainWindow::instance()->setTitle();
-}
-
 extern "C" void subsurface_mkdir(const char *dir)
 {
 	QDir directory;