Wms driver client improvements

It's really huge, again... Things have moved on the openJUMP side, so I've been forced to get there here too. I've tried to improve some weaknesses I think their client had, I really hope they can be integrated upstream.

I've tested my work again and again... with 1.1.1, 1.1.0 and 1.3.0 WMS servers. Everything seems to run fine. There are still changes to make in OrbisGIS, particularly in the UI, in order to:
- list available styles for a layer
- be able to choose a style file
- be able to give a preferred WMS version from the UI

I think namespace management may have to be improved too. Must dig to be sure, though.