Simple insets support

Simple insets support

This changeset adds the basic plumbing for supporting insets, such
that selection handles can reflect the true bounds of a widget, and
such that guidelines which show adjacent matches can show and measure
the correct visual distance between the widgets.

The actual insets data is not available (though there are some
commented out measurements for some of the most important widgets,
like buttons), so the purpose of this changeset is to put the APIs in
place such that the various view rules can correctly account for these
deltas when they become available.

Change-Id: I326bfa22f0d239d76685b371d38bb8eac594a53b