Make the error text participate in select all and copy

Make the error text participate in select all and copy

When there are rendering errors, the error messages are listed in a
StyledText widget which opens up at the bottom of the layout editor.

This changeset makes it possible to copy & paste the error message. It
does this by:

- Making the global Select All action operate on the error label when
  the error label has focus

- Make the global Copy action operate on the error label if the error
  label has selected text (which could have been made selected either
  by the Select All action above, or by a manual mouse selection)

Change-Id: Ia7f267737a0d43a54dce8cf6b5f5802aef038881