IMPROVEMENT: adding link to URLSegment Field and small code cleanups

This code change does the following:

1. it changes the URL Segment Field (CMS - Page View) Read Only Value from a span into a link that you can click on (opening the current page in a new window).

2. it also includes some small code changes to the /cms/javascript/SiteTreeURLSegmentField.js file - making it easier to read.
As pretty much every method in /cms/javascript/SiteTreeURLSegmentField.js references Field, new method getInputField was created to unify and simplify.