The trouble has been in the deleting already deleted objects. I remove all code from destructors and modify one member to pointer for assign hem parent.

Conflicts:
	Sankore_3.1.pro
	src/board/UBBoardPaletteManager.cpp
	src/board/UBBoardView.cpp
	src/document/UBDocumentProxy.cpp
	src/gui/UBDockPaletteWidget.cpp
	src/gui/UBTeacherBarWidget.h
	src/web/UBWebController.h

Visibility of palette based on children elements

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.