I'm pretty sure I'll love to use it in writerfilter instead of the normal
API whenever I get the chance.

Otherwise with malformed xml the dtor is called in a place where
the processing of the formula apparently corrupts the internal
representation somehow and it is impossible to write the document
back as docx. As a bonus, all UI elements related to saving get
disabled, so the document then cannot be saved at all. Although
this is only with malformed xml, which hopefully should never
happen, it's still better to avoid this. Moreover it seems to be
the proper way of coding the handlers anyway.

The xml parser apparently can call the characters() callback several
times for one text content (if it needs to un-escape maybe?), so
append, otherwise some content would be overwritten.

 - until we have better implementation

For cell-anchored drawing objects.  Failing to do so would result in
either one of

1) write error during file save,
2) drawing object disappearing upon reload, or
3) drawing object relocated to a wrong sheet, with wrong position
   and size upon reload.