A message for user that it is impossible to add a property to object, if it or its inheritors has elements on the scene or on the explorer of logical model.
A possibility to change the appearance of elements.
Merge branch 'master' of https://github.com/qreal/qreal into appearanceOnflight

Conflicts:
	qrgui/qrgui_ru.qm
	qrgui/qrgui_ru.ts
	qrgui/umllib/edgeElement.cpp
	qrgui/umllib/embedded/linkers/embeddedLinker.cpp
	qrgui/umllib/nodeElement.cpp
	qrgui/umllib/nodeElement.h
	qrgui/umllib/private/copyHandler.h
A first version of removing the element.
Warnings to the user when removing an element.
Add a menu item.
NxtDisplay
style changes
Added undo/redo framework base
separate widget with gridLayout
separate widget with gridlayout
Addition of element.
Added create command implementation
Added primary create/remove command implementation
Added undo stack respectivity to digram
style changes, textured buttons
images
Creating and saving of interpreted metamodel.
remove
dotMatrixDisplay