CompoundColumns test page: Add grid and form to test combinations

Allows testing with ColumnSet, ColumnHider, and/or ColumnResizer.

Also did some general formatting cleanup.
ColumnHider: break out column-showing logic to _showColumn
ColumnHider: Call resize after applying IE8 workaround
ColumnResizer: Suppress focus outline on resizer node
CompoundColumns: Add logic to support ColumnHider and ColumnResizer

Note that this currently requires that ColumnHider and/or ColumnResizer be
mixed in *before* CompoundColumns, since CompoundColumns will extend
methods from the other two.  This is in contrast to ColumnSet, which needs
to be mixed in *after* CompoundColumns.

Also includes some general cleanup.
Fix a problem with combining ColumnSet, ColumnResizer and CompoundColumns.
CHANGES.md: Note CompoundColumns interoperability w/ Hider and Resizer