Brief catches Enter press as open even if the focus is on sidebar/address/search bar

[tanriol]

https://www.mozdev.org/bugs/show_bug.cgi?id=24627 https://www.mozdev.org/bugs/show_bug.cgi?id=23706

It would be nice to move all shortcuts from the browser context to the tab context... but then it breaks. It seems that the security boundary between brief.xul and feedview.html stops us from catching the events.