Reduce the amount of false positives while detecting stereo mode

Initially the CStereoscopicsManager::DetectStereoModeByString
function considered file names containing strings like " SBS "
to be 3D content. However, it turned out this also caused false
positives, for example when opening PVR recording from the
Dutch TV channel "SBS 6".

Reduced the amount of false positives by also searching for
the keyword "3D" in the file name before considering a file
to be 3D content