* src/client/components/client-web-view.vala (ClientWebView): Don't
  require an instance of GearyApplication to be passed in to resource
  loading, use new GioUtil methods to load the resources instead. Update
  subclasses and call sites. Add a static init method for initialising
  the WebKit.WebContext move code from GearyController here.

* src/client/application/geary-application.vala (GearyApplication):
  Reimplement ::create_builder and ::read_resource using new GioUtil,
  deprecate them. Merge ::load_ui_resource_for_manager into
  ::load_ui_resource since that's its only use, deprecate it.

* src/client/util/util-gio.vala: New util methods for loading GResources,
  independent of the app.

* src/CMakeLists.txt: Add new util source.

UNit tetsing