Methods defined in widgets are available in URL instances

This is the problem that methods defined through a widget definition were not accessible from Shoes::URLs (see #890)

* Shoes::App handles method definitions and subscribers
* fixes #890

Should be good to go, would like a code review, though. :-)
It most likely is not - thanks for the input!

Might as well stay like this if I add the subscription mechanism, though (because then we don't have the explicit list anywhere anymore).