.fillable() predicate on vector paths

This deals with a minor hack in the Leaflet source by providing a trivial `{path,circle,rect,etc}.fillable()` predicate.

Moreover, it will also make it possible to clean up a certain hack in Leaflet.draw: https://github.com/Leaflet/Leaflet.draw/blob/master/src/edit/handler/EditToolbar.Edit.js#L188

Which in turn means that plugins that introduce new editing modes (I'm working on one) won't have to copy-paste and propagate that hack around. Would be much appreciated!