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

fixed issue SANKORE-574 and font manager is now translated depending on the system lanaguage (linux only but other will be fixed soon too) and some script build issues fixed on linux

Launch Sankore on desktop mode

Fix for a document's page duplication. Teacher bar's info wasn't quite correct. This is also related to ticket #409 part 2.

Fix for losing content mentioned in the ticket 228. The problem was actually in page deletion algorithm which has been flooding cache with invalid data, and that data was a object to return when loadDocumentScene was called.