QSM: Reintroduce guard argument evaluation

QSM: Reintroduce guard argument evaluation

Implements the suggestion from Simon Hausmann (codereview 89716
from 08-05 14:46) to use QQmlScriptString rather then the previous
used MetaObject-manipulation.

This also introduces comparison operators for QQmlScriptString
to be able to determinate if a QQmlScriptString changed what
is needed cause there is otherwise no way to access (all) the
needed details within QQmlScriptStringPrivate.

Change-Id: I198479eac8fd37cbdd98a99aacdd8eebf7b75d21
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>