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.h

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef APPLICATIONSTATE_H
+#define APPLICATIONSTATE_H
+
+// By using an enum class, the enum entries don't polute the global namespace.
+// Moreover, they are strongly typed. This means that they are not auto-converted
+// to integer types if e.g. used as array-indices.
+enum class ApplicationState {
+	Default,
+	EditDive,
+	PlanDive,
+	EditPlannedDive,
+	EditDiveSite,
+	FilterDive,
+	Count
+};
+
+ApplicationState getAppState();
+void setAppState(ApplicationState state);
+
+#endif