Fix wrong calls to Gtk3::main_iteration and Gtk3::events_pending

Both are functions and not methods. So call them as such avoiding
a called with too many parameters error with current
Glib::Object::Introspection
Fix call to get_computed_style on a non-element.

The top most parent of all DOM nodes is the DOM document
which is not an element. So don't try to call get_computed_style
on it.
Workaround for bug in the webkitgtk bindings

The last parameter to DOMDocument::evaluate is allowed to be undef/NULL
according to the doucmentation but not according to the introspection
files. Pass an unused object instead of undef to avoid an error message.

This fixes the last problem with Glib::Object::Introspection 0.013