rework title & icon updates

unify title update access to
- maketitle() and the need_maketitle flag for use on update
- free_titles for use on exit
- force_update_title for use after suspend/resume

additionally
- remove unused get_i11_title(), mch_can_restore_title(),
    mch_can_restore_icon(), oldtitle
- extract mch_restore_icon from mch_restore_title

inline constant return value of get_x11_title