qml: required does not work on alias properties

It seems that required alias properties do not work at runtime, see attached Main.qml file: Two properties of the component are made required: an alias one and a non-alias one. The non-alias one works at runtime and has the right value, but the alias one (when uncommented) leads to a runtime error.

Related: it seems that we error out about `required property alias hello: ...` in qmljsimportvisitor, maybe we should allow this case instead?