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 anyway:

• Choosing a previously used address should be discouraged

• When I'm already using the keyboard the 'paste address' button is

useless - just use the Ctrl-V. 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.