Add DisplayImageNumber property to DemoCamera.

This property, when set, encodes a seven-segment display into the upper-left
corner of the image, that shows the sequence number for the image.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14385 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd