Fix #383: Make ColumnSet and CompoundColumns compatible with each other

This also includes a few CSS tweaks to improve the layout of the grid header
when using ColumnSet and CompoundColumns together.

Note that ColumnSet remains agnostic to any details specific to CompoundColumns.
Keyboard: Prevent errors attempting to refocus after structure change
CHANGES.md: Note CompoundColumns + ColumnSet interop
Fix a timing problem which caused an error in unit tests in IE < 9.

This sets _started to false when a list or grid is destroyed, so that
any delayed resize handlers no longer attempt to resize when the grid's
DOM is no longer relevant.
CHANGES.md: Note change to _started behavior in List#destroy