add OverlayFillBehavior
add OverlayBackground to override the brush
add GlowBrush property
BorderBrush, BorderThickness
add CloseOnOverlay property
add AllowMove property, close only on PART_Overlay
add IsModal property
naming: OverlayBackground -> OverlayBrush
docu for the properties
add AllowFocusElement property
Merge pull request #3 from punker76/more-enhancements

more enhancements