Simplify javascript debugging by adding @sourceURL to wicket evaluations

Currently, adding breakpoints to evaluations (widely used) is impossible. This makes debugging hard.

Most debuggers (browser-based) now support the @sourceURL [1] annotation which solves this issue by exposing every eval as a file in the debugger. Note: this should only be available in wicket.development mode.

[1] http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/