support ability to define submit / validate handler arrays

For a pageroute I'm developing, I need the ability to define multiple submit and validate FAPI handlers in getDefaultValidateHandler() and getDefaultSubmitHandler() functions. However at present pageroute_insert_handlers() only supports the definition of one default handler. It'd be great if you could allow for one or more handlers to be defined in an array. The attached patch adds this functionality and is working well for me so far.

Cheers,
Stella