Only save a new draft if the current draft has not been saved.
Only save a new draft if the current draft has not been saved.

* src/client/composer/composer-widget.vala (ComposerWidget): Add
  ::is_draft_saved field, update it as the draft manager's state changes,
  or if the message has been changed. Rename ::can_save to ::should_save
  to better reflect what it is used for, don't allow saving if a draft
  has been saved. Rename update_draft_state as a proper method, use it
  update initial state after opening draft manager, tidy up location of
  draft signal handlers.

tshikaboom/geary,tshikaboom/geary,tshikaboom/geary