Allow image URL in theme manifest.json to be a relative link as well

Relative links are resolved against the manifest.json, the same way
as is done for the CSS URL.

This is needed when you are living places of the world where you
need to host everything yourself, as linking to external servers
is difficult due to privacy concerns. If your software can be hosted
on different servers, everything needs to be relative to work
properly.

js/elFinder.js

@@ -10235,6 +10235,9 @@ elFinder.prototype = {
 						if (themeObj.cssurls) {
 							themeObj.cssurls = absUrl(themeObj.cssurls, m[1]);
 						}
+						if (themeObj.image) {
+							themeObj.image = absUrl(themeObj.image, m[1]);
+						}
 						dfd.resolve(themeObj);
 					}).fail(function() {
 						dfd.reject();