apply unassigned amount to a current split

Use case

• as a user I want to add/remove split line items
• when I add/remove a split item
• then an unassigned amount becomes non-zero
• I want to be able easily to adjust an amount on another line item without using a calculator
• when I select a line item a button "Apply Unassigned" should become active
• then I can simply click on the button
• then the amount on selected item should be adjusted
• and an unassigned total should become zero