make PropertyDeclaration implicitly shared

Change-Id: Iad2f353371bd64ce8138dc86ed1f07ac994cf237
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
rename mergeItem to inheritItem

Change-Id: I21ad394076a7f2b2b2a1ff8892d427991e5613e4
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>