Multiple DatabaseTabWidget fixes.

Close databases on exit. Ask to save changes on database close.
Remove database from db list on database close.
Delete file pointer on database close.
Require to set master key at database creation time.
Append "*" to tabname if database is modified.
Handle database modified signals in database tab widget.

src/gui/DatabaseWidget.cpp

@@ -155,6 +155,11 @@ void DatabaseWidget::updateMasterKey(bool accepted)
 {
     if (accepted) {
         m_db->setKey(m_changeMasterKeyWidget->newMasterKey());
+
+    }
+    else if (m_db->transformedMasterKey().isEmpty()) { // TODO other test?
+        Q_EMIT closeRequest();
+        return;
     }
 
     setCurrentIndex(0);