Don't rely ComposerState::state notify and property bindings firing when
the value hasn't actually changed to update the composer's UI. This is
works around Vala Bug 631267 landing.

* src/client/composer/composer-headerbar.vala (ComposerHeaderbar): Add
  signal and methods to be called explicity on state changes, rather than
  relying on being notified of ComposerWidget::state changing.

* src/client/composer/composer-widget.vala (ComposerWidget): Hook up new
  ComposerHeaderbar signal and method. Remove notify and property
  bindings on ::state property, move related code to new
  ::update_composer_view method and call that as needed. Remove
  overridden ::show_all, because the UI state should now always be
  consistent with ::state. Clean up various methods.

* src/client/composer/email-entry.vala (EmailEntry::EmailEntry): Make
  widget visible by default, since we are no longer calling
  ComposerWidget::show_all().

* src/client/application/geary-controller.vala: Don't call show all on
  new composers, they now manage their visibility properly.

* ui/composer-headerbar.ui: Make toolbar icons visible by default.

* ui/composer-widget.ui: Make toolbar icons visible by default, set Send
  button icon here rather than in the class.