make Qt.core.buildVariant a list of strings

It can contain "debug", "release" or both.

Change-Id: I37cfb6033fb5a2daa02d9d1149d5c8844cb3a4b9
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>