Fix bad behavior in WebBrowser() datatype

It now binds to its real default value when
instanciated without value (or empty string value).

Example:
    >>> a = WebBrowser()
    >>> print(a)
    xdg-open
Add brwose() method to Path() type for browsing

This new method is similar to edit(), except that
it displays the file path with $BROWSER setting
instead ot editing it with $EDITOR.