ui: Better tab order in send coins entry

Pressing <tab> after entering a label now brings the focus to the address entry, instead of the row of buttons. In my experience this is more useful, as I usually want to paste an address after entering the label.

The buttons are mostly useless in this case:

- Choosing a previously used address should be discouraged

- When I'm already using the keyboard the 'paste address' button is
useless - just press Ctrl-V or Shift-Insert. Maybe it would be an idea to remove it
completely

- I usually don't want to remove the entry I'm typing now! So makes sense to have it at the end of the tab chain