WinRT: Use relative paths when calling LoadPackagedLibrary

When e.g. Qt plugins are loaded, they should be loaded relative to the
application path.

Change-Id: Ie117c4641e6aab8a4ca9fe602b7981135c9ffb13
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>