Fix recalculation of `icon-title-format' after a frame is iconified

Fix recalculation of `icon-title-format' after a frame is iconified

Previously it would only happen upon the next mode line
redisplay, meaning that just pressing C-z would not update the
implicit title, since C-z doesn't cause a redisplay.

* src/dispextern.h: Update prototypes.
* src/frame.h (SET_FRAME_ICONIFIED): De-slugify.  Call
`gui_consider_frame_title', since `icon-title-format' might be
different from the current frame title.
* src/xdisp.c (gui_consider_frame_title): Export (also in
dispextern.h).  (bug#55850)