QtIFW

let value() also query the Settings object

 if a value can not be found in the m_vars hash, value() will query
 the Settings object first before returning the default value.
 This allows to specify arbitrary tags to be defined in e.g. config.xml
 which can later be accessed by installationscripts.

Change-Id: I881ff961e3647fa32a0219bbfc8bbb89935ff3d4
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>