Make it possible to create value mappings directly on item creation

Hi Zabbix devs,

Right now, when creating a new item there is no option to create the value mapping directly from the item creation page. A lot of the times though, when building new templates you won't think about value mappings until it's too late. Which means I have to navigate back to the host/template configuration to create a value mapping, then continue with the item creation.

Would be a lot more flexible if we can create them right in the item creation page: