Cleanup: Get rid of special toString and destroy handling in QObject bindings

These are meant to go through the regular V4 binding layer

Change-Id: I23743e183b0c85cad2e047eb99dca1a9382597fb
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Port method closures over to V4

Replace the JS/V8 based closure wrappers for invokable Qt methods with
a simple QV4::FunctionObject subclass.

Change-Id: Ibd16cf8a871ce2bf0692c7c0032527109065aa8a
Reviewed-by: Lars Knoll <lars.knoll@digia.com>