Use new getproperty/setproperty syntax

[tknopp]

see: JuliaLang/julia#24960

I am not yet sure how we handle getproperty since it currently requires an output type. Can we generate that automatically (somehow)

ping @vtjnash: Do you have a concrete vision how to make use of the new feature in Gtk.jl?

[vtjnash]

I was thinking of using obj.prop[Type] / Type(obj.prop) (e.g. convert)/obj.prop = val`

But perhaps it better just to be type-unstable (for now, until we get GI codegen setup), and use simply obj.prop with introspection (g_object_class_find_property).