make switching editors work again (and a lot more efficient)

* if the widget stack is asked to switch to the already current widget -> do nothing
* hang on to the KService to see if we're loading the same thing (and if so, don't bother)
* only recreate the gui when first loading a part (removes flicker, makes things instant)
* don't create TextEditor widgets over and over for each file (they were actually leaking)
* be careful when accessing the m_textEditor pointer