Image metadata download

Download the original metadata. See https://trac.openmicroscopy.org.uk/ome/ticket/10349


The first 3 commits have been applied onto ```dev_4_4``` (see #1147)

To test:
 * Select an image. 
 * Go to the Acquisition tab and make the the Original metadata taks pane is available
 * Expand. The data should be loaded.
 * Click on the download button to download the file
 * Go back to the General tab. Click on the 5th button,
 * A menu will pop up. The "Download original metadata" should not be greyed out.
 * Click on the menu item. Download the metadata.
 * Check that the content is the same if you download it from the web