6850 loading tile in big image viewer

Test this PR on slower connections between web & server (perhaps development server is the best to test) 

1. Load a big image, grab red box in bird's eye, click and drag it around. Now, it should delay moving viewer until you stop moving mouse (you don't need to release mouse button). After 300ms viewer should move to the right location. See https://trac.openmicroscopy.org.uk/ome/ticket/6850
2. Previously every time user clicked zoom button or spin the mouse wheel all tiles were loaded. After many seconds if not a few minutes, final zoom level was eventually displayed. Now zooming event should be delayed by 300ms and only final level should request tiles.

All requests should be easily accessible on the webserver access log.