ENH: Add 'view URL' function

Add a pop-up dialog to the extension details page that provides the URL
for the extension package. This allows users looking at an extension
from within Slicer (where there is not an address bar) to obtain the URL
for the details page.
BUG: Fix URL field width on Firefox

Work around https://bugzilla.mozilla.org/show_bug.cgi?id=416634 by
adding a -moz-calc width to the URL field. This should provide an okay
width on Firefox, which does not honor absolute position of <input>
elements (see bug), while being ignored on other browsers and thus
allowing the non-hack behavior to be used instead.
ENH: Close 'view URL' dialog on outside click

Make the 'view URL' dialog modal; dimming the rest of the page is nice,
and this also allows us to trivially arrange to close the dialog when
clicking outside of it (which we also do).
Merge pull request #52 from mwoehlke-kitware/view-detail-url

Add 'view URL' function