ResourceNodes: Fix paths in copy url/path actions

Simplify qrc:/// to qrc:/, since that's the documented form. Also work
around QDir::cleanPath() not cleaning up leading '//' on windows.

Task-number: QTCREATORBUG-12299
Change-Id: Ifa75196b8682d4d9e8d5dc6045820000335b26c2
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>