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: Make sure 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).