Add support for new blur method dual_kawase [WIP]

**Work-in-Progress**

Add `dual_kawase` to configuration and argument parsing. Allow `kawase`
for backward compatibility. Add `--blur-strength` parameter for
blur-method `dual_kawase`.

Update documentation to reflect the new blur-method and parameters.

src/backend/xrender/xrender.c

@@ -507,6 +507,12 @@ void *create_blur_context(backend_t *base attr_unused, enum blur_method method,
 		ret->method = BLUR_METHOD_NONE;
 		return ret;
 	}
+	if (method == BLUR_METHOD_DUAL_KAWASE) {
+		log_warn("Blur method 'dual_kawase' is not compatible with the 'xrender' "
+		         "backend.");
+		ret->method = BLUR_METHOD_NONE;
+		return ret;
+	}
 
 	ret->method = BLUR_METHOD_KERNEL;
 	struct conv **kernels;