Add callbacks for unresponsive render process (fixes #3661)

The client can optionally wait or terminate the render process.

Expose process exit codes via OnRenderProcessTerminated and
CefGetExitCode (fixes #2126).

cefclient: Add a new https://tests/hang page for testing hang behavior.

cefclient: Move message and resource handling to a new BaseClientHandler
class to support loading of test pages in default Chrome UI windows.

tests/cefclient/cefclient_gtk.cc

@@ -110,7 +110,7 @@ int RunMain(int argc, char* argv[]) {
   // fails or if early exit is desired (for example, due to process singleton
   // relaunch behavior).
   if (!context->Initialize(main_args, settings, app, nullptr)) {
-    return 1;
+    return CefGetExitCode();
   }
 
   // Force Gtk to use Xwayland (in case a Wayland compositor is being used).