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/core/EntryAttachments.cpp

@@ -66,7 +66,7 @@ void EntryAttachments::set(const QString& key, const QByteArray& value)
     }
 
     if (emitModified) {
-        emit modified();
+        emit entryAttachmentsModified();
     }
 }
 
@@ -82,7 +82,7 @@ void EntryAttachments::remove(const QString& key)
     m_attachments.remove(key);
 
     emit removed(key);
-    emit modified();
+    emit entryAttachmentsModified();
 }
 
 void EntryAttachments::remove(const QStringList& keys)
@@ -106,7 +106,7 @@ void EntryAttachments::remove(const QStringList& keys)
     }
 
     if (isModified) {
-        emit modified();
+        emit entryAttachmentsModified();
     }
 }
 
@@ -126,7 +126,7 @@ void EntryAttachments::clear()
     m_attachments.clear();
 
     emit reset();
-    emit modified();
+    emit entryAttachmentsModified();
 }
 
 void EntryAttachments::copyDataFrom(const EntryAttachments* other)
@@ -137,7 +137,7 @@ void EntryAttachments::copyDataFrom(const EntryAttachments* other)
         m_attachments = other->m_attachments;
 
         emit reset();
-        emit modified();
+        emit entryAttachmentsModified();
     }
 }