* src/CMakeLists.txt: Pass the build dir through to the application so it
  can work out where to find the extension when running from the source
  tree. Build geary-static as reloacatable so we can link it with the
  extension. Actually link geary-static into the extension and install it.

* src/client/application/geary-application.vala
  (GearyApplication::get_web_extensions_dir): New method that determines
  where the web extension lib is to be found.

* src/client/application/geary-controller.vala
  (GearyController::open_async): Set the extension dir on the WebContext,
  and pass through logging config to the extension.

* src/client/web-process/web-process-extension.vala: Add a
  GearyWebExtension extension object, create it on init and init logging.