EphyEmbedShell and EphyWebExtensionProxy cleanups
Currently we have a wild dual-ownership situation between EphyEmbedShell and EphyWebExtensionProxy (which is probably my fault from years ago). EphyWebExtensionProxy owns itself and unrefs itself when its D-Bus connection is closed, while EphyEmbedShell just watches for the EphyWebExtensionProxy to destroy itself with weak pointers. But this is racy: sometimes the EphyWebExtensionProxy is leaked because the UI process quits before the D-Bus connection is closed. Whoops.
There's no need for objects to own themselves. It's much more natural for EphyEmbedShell to own the EphyWebExtensionProxy, since that's where it's created. EphyWebExtensionProxy can just signal when the connection is closed, so that EphyEmbedShell knows the right time to destroy it.
This MR also contains a couple general code cleanup commits, in the spirit of making it harder to review. You can examine the commits individually to better see the important bits in "Have EphyEmbedShell own EphyWebExtensionProxy."