show available download for INSPIRE atom feed in dialog

fixed some dialog issues, and added a 'Data' button, which get's enable when you have a 'INSPIRE Atom' link type available in your record.

These should link to a (geo)rss feed link, one like: http://geodata.nationaalgeoregister.nl/ahn2/atom/ahn2_uitgefilterd.xml
Which is being loaded and parsed using 'feedparser' and presented in a Data feed dialog (html), containing all links of the downloads (plus some other data).

The original idea was to show the polygons of every download IN the qgis canvas, but currently all downloads have the same extent (instead of the extent of the data).

Anyway: using the 'dutch nationaal georegister' and searching for 'ahn2' is the easiest test.
Nice thing about 'INSPIRE Atom'-type links is that you always now that you get an atom-feed. Other data types like: 'download' or 'access-link' are just to different in types to what they link...

Anyway, with me it look like this:
![screenshot from 2014-03-31 17 26 32](https://cloud.githubusercontent.com/assets/731673/2569152/dabc9f0a-b8e9-11e3-95e8-35ca95a66ce2.png)