Add unresponsive web process error page

WebKitGTK 2.33.1 allows us to detect and kill runaway web processes. Let's take advantage of that.

The process is not detected as unresponsive for quite a while -- 10 seconds -- so there's no need for any secondary timeout at the Epiphany level. Let's kill it immediately when WebKit says the web process has been lost. If the WebKit timeout were shorter, then prompting the user to decide whether to kill the page would make more sense, but 10 seconds is long enough that any web content that hasn't returned to the main loop deserves to be immediately killed. There's no excuse for that.

WIP because we don't have a successful 2.33.1 build yet.