Compile with WebKitGTK 2.3.x: Closes bgno#720315

WebKitGTK 2.4 will introduce using GClosure to register
DOM.EventTarget listeners, but old API still available, so using
that for now.

This also introduces compiling WebKit-3.0.gir to a VAPI and using
that during compilation.  This means that .gir warnings won't
cause compilation errors (--fatal-warnings) and that we have a bit
more control over binding generation.  Closes bgno#720442.