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>

core/applicationstate.cpp

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "applicationstate.h"
+
+static ApplicationState appState = (ApplicationState)-1; // Set to an invalid value
+
+ApplicationState getAppState()
+{
+	return appState;
+}
+
+void setAppState(ApplicationState state)
+{
+	appState = state;
+}
+