[TREELIST] Allocate treelist_item_t::subitems[] lazily.
[TREELIST] Allocate treelist_item_t::subitems[] lazily.

Whenever possible (i.e. there is not excessive number of columns, all
subitems of the item are NULL or MC_LPSTR_TEXTCALLBACK), wedo not need
the array.

This is especially beneficial for apps trying to save memory by using the
text callback for all its subitems.

mity/mctrl,mity/mctrl,mity/mctrl,mity/mctrl