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()`