Mark custom properties in image schema as non-base

Currently it is impossible to determine if given image schema property
is base or custom one and knowledge of that can be handy in some
situations.    Proposed change appends to every custom property special
key which determiness that it is not a base property.

Change-Id: I49255255df311036d516768afc55475c1f9aad47
Partial-Bug: #1371559
Merge "Mark custom properties in image schema as non-base"