This gives us a means of using JS objects returned by UserContentManager
script messages directly from Vala.

* bindings/metadata/WebKit2-4.0.metadata: Tweak standard webkit2 VAPI to
  re-include methods that return javascriptcore objects.

* bindings/vapi/javascriptcore-4.0.vapi: Copy generated VAPI file into
  the tree, customise it so that it actually works.

* src/CMakeLists.txt: Generate the custom webkit2 VAPI, depend on
  javascriptcore and include in-tree javascriptcore VAPI in the client
  build.

* bindings/metadata/JSCore-3.0.metadata: Remove obsolete file.