Qt4BaseTargetFactory: Expose list of generic 'features' for a target

Replace the isMobileTarget, supportsShadowBuilds methods by a
generic targetFeatures method which exposes features as a set of
strings.

Change-Id: I2ac99ff5a82280316071efaa140cd7ea35b21374
Reviewed-on: http://codereview.qt.nokia.com/692


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>