Partial updates for ubuntu 20.04

[milesfrain]

Looks like gir1.2-webkit-3.0 on bionic (18.04) was updated to gir1.2-webkit2-4.0 on focal (20.04).

This PR is an initial attempt to apply those updates; however, a more involved rewrite is required to incorporate the WebKit2 API. For example, there's no longer a set_transparent method or "status-bar-text-changed" signal.

I'm not going to be spending any more time on this update, but wanted to share some progress so far in case anyone else wants to continue effort.

I'm testing this by running bin/ojo as described in the contributing instructions.

[peterlevi]

Thanks, @milesfrain. I've had it on my radar to make the necessary research and fixes to make Ojo work on Ubuntu 20.04+.
One of the bigger issues is exactly that "status-bar-text-changed" signal - we use it for all the Python<->JS communication, it's essential, but there seems to be no decent equivalent to this in Webkit2 4.0.

Help from someone more familiar with Webkit 4.0 appreciated on this PR.