No system URI handling

[xerz-one]

When opening a link within Squeak after installing MagicMouse, Morphic will complain that "There is no Web Browser installed", as there's no registering as WebBrowser, no property #webBrowserView set to true, and no method isWebBrowser returning true.

As an example of how to implement system URI handling support, take Scamper:

https://github.com/hpi-swa-teaching/Scamper/blob/master/packages/Scamper.package/Scamper.class/class/initialize.st https://github.com/hpi-swa-teaching/Scamper/blob/master/packages/Scamper.package/Scamper.class/instance/morphicWindow.st https://github.com/hpi-swa-teaching/Scamper/blob/master/packages/Scamper.package/Scamper.class/instance/isWebBrowser.st