QSystemFeaturesInfo API

Essentially this is to improved the extendability of the features requirement, while keeping compatibility. This will create a new class QSystemFeaturesInfo .
For compatibility, we should keep the current QSystemInfo::hasFeatureSupported.

This new class has function hasFeatureSupported would take a QString as a parameter.
For already existing features, it can use old QSystemInfo::hasFeatureSupported. (or move those implementations into this class, and use QSystemFeaturesInfo from QSystemInfo.

• default supported parameters would be documented
• add a method which lists the available parameters
• Add errorLevel() method to tell whether the last function call succeeded
• Possible return codes could be at least NoError, Error, NotSupported