Added mountType attribute and adapted Delete action text

Added mountType attribute for files/folder to indicated whether they are
regular, external or shared.

The client side then adapts the "Delete" action hint text based on this
information.

Only the mount roots must have the delete icon hint adapted.
To make this detectable on the client side, the mountType can now
be null, "shared", "shared-root", "external" or "external-root".

This also gives room to icon customization on the client side.