PreferencesDialog: New settings for font size of SQL editor and log

Add two new settings to allow changing the font size inside the SQL log
and the SQL editor.

Increase the default font size from 8 to 9 - better a bit too big on
some systems than too small on others.

Also do some restructuring in the MainWindow to avoid duplicate code to
reload settings.

src/MainWindow.h

@@ -144,6 +144,7 @@ private slots:
     virtual void openSqlFile();
     virtual void saveSqlFile();
     virtual void loadExtension();
+    virtual void reloadSettings();
 };
 
 #endif