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/CMakeLists.txt

@@ -37,6 +37,8 @@ endif()
 
 # compile the core library part in C, part in C++
 set(SUBSURFACE_CORE_LIB_SRCS
+	applicationstate.cpp
+	applicationstate.h
 	checkcloudconnection.cpp
 	checkcloudconnection.h
 	cloudstorage.cpp