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/gui/dbsettings/DatabaseSettingsDialog.h

@@ -21,8 +21,9 @@
 #include "gui/DialogyWidget.h"
 #include "config-keepassx.h"
 
-#include <QScopedPointer>
 #include <QPointer>
+#include <QScopedPointer>
+#include <QSharedPointer>
 
 class Database;
 class DatabaseSettingsWidgetGeneral;
@@ -47,7 +48,7 @@ public:
     ~DatabaseSettingsDialog() override;
     Q_DISABLE_COPY(DatabaseSettingsDialog);
 
-    void load(Database* db);
+    void load(QSharedPointer<Database> db);
     void showMasterKeySettings();
 
 signals:
@@ -66,7 +67,7 @@ private:
         Security = 1
     };
 
-    QPointer<Database> m_db;
+    QSharedPointer<Database> m_db;
     const QScopedPointer<Ui::DatabaseSettingsDialog> m_ui;
     QPointer<DatabaseSettingsWidgetGeneral> m_generalWidget;
     QPointer<QTabWidget> m_securityTabWidget;