Make it possible to have 'Check' functions called only once

For all sections except [Types] and [Components], Setup might call each Check function several times, even if there's only one entry that uses the check function.

Add an option to have the function called only once, for example using a CheckOnce parameter or checkonce flag.

This would cause Setup to “remove” the entry at startup when the function returns False, just like it already does for [Types] and [Components].