AUI-1683 Initial form builder screen
AUI-1683 Source formatting
AUI-1683 Hiding/showing empty msg instead of appending/removing

This guarantees the position of the message, and is less expensive.
AUI-1683 More tests for initial screen

Tests showed a bug where we were reusing the same Layout instance between different FormBuilder instances. We need to use valueFn instead of value for default attribute values.

Also noticed that we needed to update the empty message when the layout changed as well, instead of just when the layout row's changed.