Improved performance of loading text from old format.

Undo stack disabled ad scene loading because it is not needed there.

This is not functional.

Now text size in TextItem is the same on separate platforms.

used writecharacter instead of writeCDATA bacause writeCDATA leads to a file trucation when we copy text from libreoffice and the pase is saved without any changes on the text

First, lost of text formating. New format and then lost of text.

Frame buttons

Fix to Sankore 636. Now copyes of video and audio items stops in the sate tomes when stops original items.

Generating of missing thumbnails no more crashes Sankore.

Improved performance of navigating between pages.