Always bind the return key to the default button

Always bind the return key to the default button

If a dialog/window has a default button registered not every
platform associates the return key with that button, but all
users do.  We have to register the binding of the return key
ourselves to make sure the user's expectations of pressing
return will activate the default button are met.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>