Explicitly display valuation method in Item

[asbasawaraj]

While creating an Item, the Valuation Method field has three possible values in the dropdown blank, FIFO and Moving Average.

Also, the value can be set only once.

If it's left blank and the form is saved, the field disappears from the view and the 'Default Valuation Method' in Stock Settings is used for the Item.

Need to fetch the default Valuation Method and add it on the Item on Save. This would help the user to know which valuation method is used in the future.