Beautier dummy URL

By using a different value for identification of requests,
the dummy URL can be kept as simple as possible.
Remove need for robwu.nl/204 permission

Although never reached, it may feel uncomfortable to add `*://robwu.nl/204*`
to the "permissions" section of the manifest file.

This change requires "getFrameId" to be whitelisted though. This allows
other extensions to detect the presence of the extension by using

var x = document.createElement('img');
x.onload = function() {  alert('Extension is installed!');      };
x.onerror = function() {  alert('Extension is not installed!'); };
x.src = 'chrome-extension://mdjlncphegdkccpdaboajpkaiinaiipg/getFrameId?';

If this feature is undesirable, undo this commit.