Update 'renderHeaderCell' documentation
Add shouldObserveStore flag to allow electing not to observe stores
Merge branch 'master' into dev-0.4

Conflicts:
	doc/components/core-components/Grid.md
Merge branch 'master' into dev-0.4

Skip 1fd7594e (will commit 0.4 version instead)
Add shouldTrackCollection flag to allow electing not to track collections
_StoreMixin: Don't revert data when re-filtering or re-sorting