Selected non-default text class does not render after edit and received update from backend

[proycon]

reported in #170 by @pirolen:

After submitting entity annotations, the sentence where it belongs to fully disappears from the text area of FLAT.
(I attach a screenshot -- the html is there, but invisible -- hovering over the area shows the infos/added annotation.)

For the text to reappear, I have to reload the page (or the doc), or change back from the "current" text class -- if it is getting available, sometimes, but in those cases also with 'invisible' html -- to the original text class ("OCR") .

It seems something was missed after implementing #139

[proycon]

i reproduced this. the 'html' part of the response returns the structure but no placeholders for labels and no text.

<div id=\"FA-b1_3_1_mwtext_vorbemerk_pp61_67_001.text.div1.p40.s.1.w.1\" class=\"F w deepest\">&nbsp;</div>

Rendering text should have been handled by rendertextclass() but this does not get called after the partial update (which is probably the bug)