Zoom reset

Restore default thumbnail when clicking on the reset button
 
To test:
 * Browse a dataset.
 * go to the status bar of the browser (central panel)
 * Click on the +/- button to zoom in/out
 * Click reset. The size should be reset.
 * Now modify the size using the slider
* Click reset. The size should be reset.

See https://trac.openmicroscopy.org.uk/ome/ticket/11312