Improved performance of loading text from old format.

If warnings was appeared - dialog with warnings details appeared. Each warning is a separate QTextEdit.

That mechanism is general for UBExportAdaptor and can be simple implemented and used in any import or export adaptor.

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

UBToolWidget API has been deactivated, there is certainly a lot of bugs, code is ugly.
Remember to fix commented sections (forgotten which ones).

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.

PDF : Sankore overwrite Pdf layer to empty, after picture(shape) was "set as background"

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.