Proposal to rename some Morphic classes

My other Morphic changes have not caused any obvious push back, so I'd like to go ahead with my suggestion from a while back...

Motivation: When I first used Magritte, I was terribly confused that #asMorph does not actually return a morph, and requires "object asMorph morph" to get the actual morph (http://forum.world.st/Customizing-Magritte-Morphic-Forms-tt4251518.html#a4251623)

Solution: Rename classes messages to reveal intention
1) #asMagritteMorph
  a. change #asMagritteMorph to something like maMorphicContainer (returns a presenter/builder, not a Morph)
  b. if we want #asMagritteMorph, have it return the actual morph (i.e. "self maMorphicContainer morph")
2) MAContainerMorph and friends -> MAMorphicContainer - these are not Morphs. I was actually astonished when I saw they did not inherit from Morph.

If there is no opposition, I'll make these changes shortly.

BTW: what is the policy re non-backward compatible changes? How should they be documented? In Pharo, methods are deprecated for a version, but I didn't want to do that because I assumed Magritte should run on other Smalltalks. Although I see Squeak has #deprecated:, so maybe that would be safe...