Load SQLite extensions earlier

In the Preferences dialog we allow the user to configure a list of
SQLite extensions which should be loaded whenever a new database file is
create or an existing database file is loaded. This commit changes the
order of actions after creating or opening a file so that the extensions
are loaded significantly earlier. This way they are already loaded for
most of the time during the process.

This fixes some subtle bugs. One of the more prominent ones was
triggered when there is a view which uses some function which is
defined in such an extension. After loading the file, parsing would fail
and you couldn't see the fields of the view. Browsing it would work but
trying to edit the display format crashed the application.

src/MainWindow.h

@@ -184,7 +184,6 @@ private:
     void addToRecentFilesMenu(const QString& filename);
     void activateFields(bool enable = true);
     void enableEditing(bool enable_edit);
-    void loadExtensionsFromSettings();
     void saveAsView(QString query);
     void duplicateRecord(int currentRow);
     void selectTableLine(int lineToSelect);