use Escape key to close Reverse Dependencies details popup window

[juhp]

Firstly a big thanks for integrating the reverse dependency info on Hackage.

I think a really nice small improvement would be to allow closing the popup using the Escape key (that already works for the "Search in package" dialog).

[juhp]

Any pointers or hint for this would be welcome.
I am lost where to start - I tried grepping the source but got nothing so far.