QmlJS: Add more value_casts.

And switch all existing dynamic_casts to value_casts.

Change-Id: I93b89358e4802080f40b332074c64f4e91a2bc4c
Reviewed-on: http://codereview.qt-project.org/6311


Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>