add an isNumber() method to Element
update Attribute
add attribute appearance setters, behavior flags renamed, AttributeFactory is not a friend class of Attribute
update attribute
new AttributeSet class
Merge branch 'master' of https://github.com/KiwiMusic/KiwiCore

Conflicts:
	KiwiBase/Tag.cpp
	KiwiBase/Tag.h
	KiwiBoxes/Arithmetic.cpp
add frozen_attributes tag