This allows removing a large number of uses of the apps's static instance
variable, which is highly desirable.

* src/client/application/geary-controller.vala (GearyController): Add
  GearyApplication as a weak internal field. Set it in the ctor. Use that
  instead of GearyApplication.instance throughout.

* src/client/application/geary-application.vala
  (GearyApplication::controller): Pass this in to the controller when it
  is constructed.