CompoundColumns test page: Add grid w/ ColumnHider + ColumnResizer

Also general formatting cleanup
ColumnHider: break out column-showing logic to _showColumn
CompoundColumns: Add logic to support ColumnHider

Note that this currently requires that ColumnHider be mixed in *ahead*
of CompoundColumns.

Also includes some general cleanup.
ColumnResizer: Check that a column is passed to resizeColumnWidth

It is possible for resizeColumnWidth to receive an undefined column value
when used with CompoundColumns (since the parent spanning cells do not
exist in the column structure).
ColumnHider: Call resize after applying IE8 workaround
ColumnResizer: Suppress focus outline on resizer node
CompoundColumns + ColumnHider: Avoid skew from redundant hide/show calls
CompoundColumns + ColumnHider: Avoid quirks in Firefox
CompoundColumns + ColumnResizer: Report proper grid width / last column

This also includes an extension to renderHeader to ensure that headerNode
is annotated on the objects in the columns object and subRows array.