Download image and metadata

In this PR:
* Modify sequence to load archived image. Now it is possible to name the file. Default if the image's name
* Add download original metadata option to menu.
* Review code to load original metadata. The image's id is specified (prep work related to https://trac.openmicroscopy.org.uk/ome/ticket/10349)


To test loading of archived image:
 * Import an image. Make sure to select the archive option.
 * Click download. The image will be downloaded.
 * Click download again. Do not change the name. A warning dialog should pop up

To test metadata:
 * Make sure that the image has a original_metadata file (not all files have one). Gatan should have one.
 * Click on the Download metadata option. The metadata file (if it exists) will be downloaded.
 * Click download again. Do not change the name. A warning dialog should pop up
![downloadMenu](https://f.cloud.github.com/assets/1022396/373602/6037f8bc-a385-11e2-92dd-1fb2a9692c9d.png)