Ensure WebKit web process cleanup.

This change manually removes the connected signal by their id for
gotk3 and go-webkit2 objects for HTML pages when a tab is closed.
This prevents a leak where connected signals can prevent finalizers
from running (see https://github.com/conformal/gotk3/issues/55).

Also, rather than waiting for the glib.Object finalizer on each
wk2.WebView (which may take up to 3 minutes before the Go GC runs),
explicitly .Destroy() the webview widget.  This causes the
WebKitWebProcess to be immediately closed.