* src/client/composer/composer-web-view.vala (ComposerWebView): Add new
  ::command_stack_changed signal to manage undo/redo enabled state, hook
  it up to a JS callback. Add ::is_empty property as a non-functioning
  shim in lieu of can_undo going away. Remove ::can_undo and ::can_redo,
  replace them with methods to actually fire an undo and redo.

* src/client/composer/composer-widget.vala (ComposerWidget): Use
  ClientWebView::is_empty rather inplace of can_redo for determining
  editing state. Remove old undo/redo signal hookups, replace with
  ::command_stack_changed and manage action enable state from there.
  (ComposerWidget::action_entries): Add explicit actions for undo/redo,
  since they need some custom code over on on the web process side.
  (ComposerWidget::on_compose_as_html_toggled): Explciitly manage the
  visibility of rich text toolbar buttons, don't rely on obscure
  GtkBuilder magic that Glade doesn't support.

* ui/composer-web-view.js: Add a mutation observer for the message body
  and explcit methods for firing undo/redo commands, so we can keep track
  of how the command stack changes. As it does, fire commandStackChanged
  messages back to the client process. Explicity set the message body as
  content-editable after the document has been mutated for quotes, etc.

* ui/composer-widget.ui: Add bonus undo/redo toobar buttons for the
  composer.