Fix edge case in signal emission semantics

Fix edge case in signal emission semantics

Signal emission triggered by property changes during an onDestruction
handler was previously not well tested.  This commit adds several
unit tests to ensure correct behaviour in that situation.

Those unit tests showed a problem in signal emission related to when
children objects are cleaned up.  This commit also ensures that if
such children own their own context, their onDestruction handlers
are called prior to marking the child as deleted.

Change-Id: Ibf84ae56ba1134e5d6402b742aee1bdc0e5e4e15
Reviewed-by: Matthew Vogt <matthew.vogt@nokia.com>