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/EditWidgetIcons.h

@@ -71,7 +71,10 @@ public:
 
     IconStruct state();
     void reset();
-    void load(const QUuid& currentUuid, Database* database, const IconStruct& iconStruct, const QString& url = "");
+    void load(const QUuid& currentUuid,
+              QSharedPointer<Database> database,
+              const IconStruct& iconStruct,
+              const QString& url = "");
 
 public slots:
     void setUrl(const QString& url);
@@ -97,7 +100,7 @@ private slots:
 
 private:
     const QScopedPointer<Ui::EditWidgetIcons> m_ui;
-    Database* m_database;
+    QSharedPointer<Database> m_db;
     QUuid m_currentUuid;
 #ifdef WITH_XC_NETWORKING
     QUrl m_url;