Previous multi-sub-element linking (e.g. Face, Edge) is supported
through SubElements of type PropertyStringList, which does not support
geometry element update tracking. This patch changes it to use
PropertyXLink's new multi-subname capability.
This patch includes the actual implementation of Link, which is
implemented as an extension named LinkBaseExtension in App namespace,
and a full view provider ViewProviderLink in Gui. The reason of not
using ViewProviderExtension is because it need full control when
display, not just extending existing functionalities.
Please see [here](https://git.io/fjPue) for more details of the
implementation.
This patch also includes a set of link manipulation commands, and a
task panel for overriding geometry element colors.
