code cleanup: rename BKE_tessmesh -> BKE_editmesh, rename EditDerivedBMesh.tc -> em. ('tc' is odd name which isn't used elsewhere).


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56022 954f8c5b-7b00-dc11-b283-0030488c597c