fix: pos item cart dom updates

[nextchamp-saqib]

Problem:

• Changing discount from POS screen, doesn't update the Grand Total, Net Total etc

Why?

• If discount is applied on the invoice, grand_total, net_total and other fields on the pos invoice are changed.
• Those change doesn't trigger an event. For eg. frm.doc.grand_total = 600 doesn't triggers grand_total event on the form
• Hence it not possible to listen for these updates to change the Cart dom.

Fix:

• On any field change of the POS Invoice, the frm.refresh_field event is manually triggered almost in every case.
• Hence, listening for that event and triggering cart refresh solves the issue.

[coveralls]

Coverage remained the same at 42.844% when pulling 7bd4c6f on nextchamp-saqib:pos-item-cart-update-fix into a8b6cf8 on frappe:develop.