Depend on javascriptcore, generate webkit2 VAPI that uses it.

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.

bindings/vapi/javascriptcore-4.0.vapi

@@ -0,0 +1,15 @@
+/* javascriptcore-4.0.vapi. */
+
+[CCode (cprefix = "JS", gir_namespace = "JavaScriptCore", gir_version = "4.0", lower_case_cprefix = "JS_", cheader_filename = "JavaScriptCore/JavaScript.h")]
+namespace JS {
+
+	[CCode (cname = "JSGlobalContextRef")]
+    [SimpleType]
+	public struct GlobalContext {
+	}
+
+	[CCode (cname = "JSValueRef")]
+    [SimpleType]
+	public struct Value {
+	}
+}