Enable disabled simplification code after merge MTR (1-2)

Disable simplification of geometries in composer by default
Enable disabled simplification code after merge MTR (2-2)

Enable AntialiasingSimplification
[layertree] Add note about used custom properties
Merge remote-tracking branch 'alvaro/Simplification_MTR'

Conflicts:
	python/core/qgsmapsettings.sip