Send dev=desktop parameter when searching (bug 1060062)

Together with https://github.com/mozilla/fireplace/commit/914a5f4b4ca27ee2ec0bf745681a8c0e67298f56
and https://github.com/mozilla/zamboni/commit/dbd97240305cc7b4e544bb739b84cb03721f42d3 this restricts
the about:app search to desktop apps only.