screens/item_details: implement resizing

[tmplt]

Just alike multiselect_menu::on_resize(). This way the user does not have to reopen an item_details screen after a resize event.

[tmplt]

#12 should be tackled first.

EDIT: done.

[tmplt]

For a resize, we'd need to know the size of the index menu. This would (and should only) be possible to know if we query the screen butler, but implementing this would be another two-way communication shoved in there just there for such a small thing.