Refactor GimpImageWindow session management. As far as session
management goes, there are four states and four transitions that are
interesting.

The states are:

 State    Window mode     Images opened
 ------------------------------------------
  s1          swm               0
  s2          swm              >0
  s3          mwm               0
  s4          mwm              >0

The transitions are:

  Transition     Description
 -------------------------------------------
      t1         Enter single-window mode (swm)
      t2         Enter multi-window mwm (mwm)
      t3         Open an image
      t4         Close an image

When thinking of it like this, it is pretty easy to see when to call
gimp_image_window_session_clear() and when to call
gimp_image_window_session_apply(). So kill
gimp_image_window_set_entry_id() and handle all transitions in
gimp_image_window_session_update().