add utility class for Overlay: OverlayObject in overlay_utils.cpp
use OverlayObject in Plotter2D
use OverlayObject in PieChartDisplay
use OverlayObject in OverlayTextDisplay
use OverlayObject in OverlayCameraDisplay
use OverlayObject on OverlayImageDisplay
fix the timing to initialize OverlayObject in OverlayCameraDisplay
use OverlayObject in OverlayDiagnosticDisplay
use OverlayObject in OverlayMenuDisplay
dispose material when deleting OverlayObject
Merge pull request #128 from garaemon/use-overlay-object-common-class

Use overlay object common class in overlay displays