PatternOverlay: child objects represent their overlay as an ImageJ Roi object instead of overlay, paving the way to have this play nicer with the scale bar overlay and possibly allowing multiple patterns to be superimposed

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14351 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd