#283 Export function should export metadata as well

[John]

Currently, when you export a xena file using the xena viewer, the base64 encoded portion of the xena file is converted to a 'proper' file. All of the xml metadata wrapped around that file is ignored.

It would be useful if the export function also exported the metadata. This could be an optional step.

As an example, take a TIFF file. Xena converts that tiff to a png file, extracts the tagged metadata from the original and adds it to the resulting xena file. If we export the encapsulated png file, it would be useful to also export the metadata - giving :
originalfilename.tif.png and
originalfilename.tif.metadataextract.txt