[Patch] Add attribution getters to ImageryInfo

[Don-vip]

I need to access attribution fields of ImageryInfo from a plugin.
Please find attached a patch that adds the needed getters.

I am currently accessing them in a very dirty way :D

			TMSTileSource tms = new TMSTileSource(null, null, 0);
			imagery.setAttribution(tms);
			... tms.getAttributionText(0, null, null));//FIXME
			... tms.getAttributionLinkURL());//FIXME
			... tms.getTermsOfUseURL());//FIXME

[simon04]

In [4464/josm]:

fix #6893 - add attribution getters to ImageryInfo (patch by Don-vip)