[qmf] Fix double emiting of messageUpdate/messageAdded signal.

Double emition was introduced by this commit:
https://qt.gitorious.org/qt-labs/messagingframework/commit/9aca2eb83a164d3da4bce635eca58e9f3751ca95
When data or properties are updated in the store instance respective messageUpdate/messageAdded signals
are already emitted.
Added some UT's to check these signals in a separated commit in this PR.