Port the KexiDataSourcePage to the new style, away from KexiPropertyPaneViewBase, update KexiFormPart
+ remove the KexiDataSourcePage FADE, it's in main window now
+ to update data source page, use the form's data source / plugin id, not the properties because the property set may belong to selected widget, not the form
+ overload propertySetSwitched() to updatte data source page when needed