Refactor Database and Database widgets (#2491)

The Database, DatabaseWidget, and DatabaseTabWidget classes share many responsibilities in inconsistent ways resulting in impenetrable and unmaintainable code and a diverse set of bugs and architecture restrictions. This patch reworks the architecture, responsibilities of, and dependencies between these classes.

The core changes are:

* Move loading and saving logic from widgets into the Database class
* Get rid of the DatabaseManagerStruct and move all the information contained in it into the Database
* Let database objects keep track of modifications and dirty/clean state instead of handing this to external widgets
* Move GUI interactions for loading and saving from the DatabaseTabWidget into the DatabaseWidget (resolves #2494 as a side-effect)
* Heavily clean up DatabaseTabWidget and degrade it to a slightly glorified QTabWidget
* Use QSharedPointers for all Database objects
* Remove the modifiedImmediate signal and replace it with a markAsModified() method
* Implement proper tabName() method instead of reading back titles from GUI widgets (resolves #1389 and its duplicates #2146 #855)
* Fix unwanted AES-KDF downgrade if database uses Argon2 and has CustomData
* Improve code

This patch is also the first major step towards solving issues #476 and #2322.

tests/gui/TestGui.h

@@ -26,6 +26,7 @@
 #include <QObject>
 #include <QPointer>
 #include <QScopedPointer>
+#include <QSharedPointer>
 
 class Database;
 class DatabaseTabWidget;
@@ -88,7 +89,7 @@ private:
     QScopedPointer<MainWindow> m_mainWindow;
     QPointer<DatabaseTabWidget> m_tabWidget;
     QPointer<DatabaseWidget> m_dbWidget;
-    QPointer<Database> m_db;
+    QSharedPointer<Database> m_db;
     QByteArray m_dbData;
     QScopedPointer<TemporaryFile> m_dbFile;
     QString m_dbFileName;