Allow properties to have unsettable int and uint fields

Allow properties structs to contain exported int and uint fields,
but only if they are tagged with blueprint:"mutated".  These fields
will be copied by CopyProperties in order to propagate through
cloning during mutations, but can only be set by Mutators and not
by Blueprint files.

Change-Id: Ic462385799c77667e392f4928d02d424c8f8d900