WIP: Changes the default targetStore of UndoManager to getParent instead of getRoot

[robclouth]

Fixes issue #832. I think that for more flexibility the targetStore should default to the parent not the root. This allows you to have undo managers on child object, for example a separate undo history for each note in a note-taking app.

Tests pass.

lerna success run Ran npm script 'test' in packages:
lerna success - mobx-state-tree
lerna success - mst-example-bookshop
lerna success - mst-example-boxes
lerna success - mst-example-redux-todomvc
lerna success - mst-example-todomvc
lerna success - mst-middlewares
✨  Done in 28.53s.

[robclouth]

No, no this doesn't work at all. We need more tests.

[mweststrate]

@robclouth added WIP status to the title to make that clear :)

[mweststrate]

I think #875 is the proper fix for this, correct @robinfehr ?

[xaviergonz]

Closing since the issue that this PR wanted to address seems to have been fixed for a long time