Unbreak "download this document" button in FF extension

getUserData/setUserData was deprecated and is no longer available in latest
Firefox (e.g. 27.0). Using e.g. the dataset property instead did not work
for storing callback methods. As currently only one async requests without
a callback are used, any support for callbacks or sync requests was simply
removed. Proper implementation delayed till callbacks or sync requests are needed.
Rename Firefox add-on to "ODF Viewer"