Rewrite selectedContainer to handle non-collapsed selections.

Includes documentation and tests.
Modify indent/outdent to use get$CommonParent

Instead of selectedContainer for premanipulation fix call.
Remove extraneous commented out code from selectedContainer
Some more non-collapsed selection tests.
Better docs for selectedContainer.
Clearer selectedContainer documentation.
Better test name for collapsed selection DDT
Better test name for some test.
Better docstring for .selectedContainer
Even better docstring for `.selectedContainer.`
Merge pull request #630 from wymeditor/selectedContainer

`.selectedContainer` with non-collapsed selections