Fix some compiler warnings

This set of patches mainly deals with the fact that there are some depreciation warnings regarding used gimp functions. There are two issues - the way xsane registers as a plugin needs to be updated (otherwise there are warning messages when gimp starts) and there are some depreciation warnings when compiling non-gegl gimp functions. I'm afraid to not break old code, this patch comes with a lot of #if lines. But we would not want to break code for old gimp versions? Other compiler warnings I fix are some trivial cases where function declaration did not match function definition.