[patch] Add support to send cookies for WMS images

[xeen]

Adds the possibility for WMSPlugin to send cookies with each Image request. The feature isn't advertized anywhere except in the code and works by adding wmsplugin.url.NUMBER.cookies=XXXX in the Einstein dialog. The patch doesn't allow to define cookies for the default sources.

The patch also cleans up some other things, i.e. removes the "handing down" of variables through several layers when they're available via the WMSLayer anyway.

Some WMS servers require settings to be sent via cookie and won't accept get parametres. This patch "works around" this limitation by allowing JOSM to send the required cookies and thus make these WMS servers useable for JOSM. (I have a specific one in mind, in case you wonder. I posted the patch because it's quite large and re-applying it everytime the WMSPlugin updates will be quite a lot of work.)

Warning: Includes the patch in #2718

[stoecker]

Applied.