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

Cannot rely on the focus out event (on Mac) in the
situation where a snippet is added/edited and the
dialog is immediately confirmed.

Reviewed-by: con
Task-number: QTCREATORBUG-3464

Reviewed-by: hjk

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