Internally replace a TextEntity with a "lighter version", that stores the raw UTF-16 data of the text.
This way, we can save about 4 int's for each text entity; this is not much for small documents,
but with big documents with lots of text (eg, the PDF specs) we can save a lot (more than 50MB!).

CCBUG: 161213

svn path=/trunk/KDE/kdegraphics/okular/; revision=803048