Turn application state into enum

The application state was encoded in a QByteArray. Thus, there was
no compile-time checking. Typos would lead to silent failures.

Turn the application state into an enum. Use the enum-class construct,
so that the values don't polute the global namespace. Moreover,
this makes them strongly typed, i.e. they don't auto-convert to
integers.

A disadvantage is that the enums now have to be cast to int
explicitly when used to index an array.

Replace two hash-maps in MainWindow to arrays of fixed sizes.

Move the application-state details into their own files.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/filterwidget2.cpp

@@ -161,7 +161,7 @@ void FilterWidget2::clearFilter()
 
 void FilterWidget2::closeFilter()
 {
-	MainWindow::instance()->setApplicationState("Default");
+	MainWindow::instance()->setApplicationState(ApplicationState::Default);
 }
 
 void FilterWidget2::temperatureChanged()