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.

src/cli/Extract.cpp

@@ -104,7 +104,8 @@ int Extract::execute(const QStringList& arguments)
 
     KeePass2Reader reader;
     reader.setSaveXml(true);
-    QScopedPointer<Database> db(reader.readDatabase(&dbFile, compositeKey));
+    auto db = QSharedPointer<Database>::create();
+    reader.readDatabase(&dbFile, compositeKey, db.data());
 
     QByteArray xmlData = reader.reader()->xmlData();