rename BastTextEditor->BaseTextEditorWidget, BaseTextEditorEditable->BaseTextEditor
rename BaseTextEditor{,Widget} subclasses
rename editableInterface->editorInterface
rename createEditableInterface->createEditor
minor cleanups after renamings

Reviewed-by: hjk

Groups are no longer enum values but identified from snippet providers.

Reloading and synchronizing are now directly accessible from
the collection interface.

Provide an interface so users can create/edit/remove snippets.