Docs: Move util/mouse replacement notes into 0.4 Migration Guide

Remove util/mouse from doc TOC but leave a placeholder page pointing to the
0.4 Migration Guide
Documentation tweaks and updates

* Revise wordings
* Remove CHANGES.md and its mention in docs/README.md
* Remove "as of dgrid 0.3.x" and Dojo-1.7-specific comments
* Reduce wording in Tree docs to mention dstore/Tree
* Add notes about dgrid-autoheight to Styling page
* Update API comparison and include links to other doc pages
* Eliminate any remaining column plugin mentions