Right now, only the arrow is clickable and that's a small target.

For info, I'm working on bug 913014 which aims at doing the same for the computed styles view. Also, bug 855523 should land shortly, and this one aims at doing the same too for the markup view. So it makes sense, for consistency reasons, whenever we have a horizontal container with an expand/collapse twisty, that the whole container also serves as an expand/collapse element. Having said this, both the markup and computed views are going to have it on dblclick, not click. Only the twisty is single clickable.

This patch allows users to dblclick on the pseudo element header itself. The twisty remains clickable as before. This patch also contains an attempt for a mochitest browser test that I haven't been able to finalize. It's basically using the `EventUtils.sendMouseEvent({type:"dblclick"}...)` API which doesn't seem to work at all for me. Any help about why that might be would be great!

Uploading a new version of the same patch after having finally found a way to simulate double click events.

Thank you Mike for the review!