Don't mess with content-related things when prepareForReuse is called
Be a little less trigger-happy when invalidating the layout
Remove an unused ivar
Don't attempt to show a header if we don't have a title for it
Make sure to update the number of columns once a row is filled

Fixes a bug where a full-row cell would be added immediately after the spanning cell in some cases.
Use the correct parameter
Display the category's name as its title, not the identifier