Share per-window WebkitGTK WebProcesses
Now that WebKitGTK has deprecated the shared process model, the only way to share a WebProcess between views is to use the
related-view constructor property.
This makes ConversationWebView use
related-view, removes the workaround needed for WebKitGTK 2.24 and the old shared process setup for earlier versions, ensures a single WebProcess is used for conversations per MainWindow - improving initial conversation selection responsiveness, and also rolls up !232 (closed), lazily constructing web views for ConversationMessage widgets until needed - improving loading perf for large conversations.