LibIPC+WindowServer+LibGUI: Detect and highlight unresponsive GUI apps

IPC::ClientConnection now tracks the time since the last time we got
a message from the client and calls a virtual function on itself after
3 seconds: may_have_become_unresponsive().

Subclasses of ClientConnection can then react to this if they like.

We use this mechanism in WindowServer to send out a friendly Ping
message to the client. If he doesn't Pong within 1 second, we mark
the client as "unresponsive" and recompose all of his windows with
a darkened appearance and amended title until he Pongs us.

This is a little on the aggressive side and we should figure out a way
to wake up less often. Perhaps this could only be done to windows the
user is currently interacting with, for example.

Anyways, this is pretty cool! :^)

Libraries/LibGUI/WindowServerConnection.h

@@ -74,6 +74,7 @@ private:
     virtual void handle(const Messages::WindowClient::UpdateSystemTheme&) override;
     virtual void handle(const Messages::WindowClient::WindowStateChanged&) override;
     virtual void handle(const Messages::WindowClient::DisplayLinkNotification&) override;
+    virtual void handle(const Messages::WindowClient::Ping&) override;
 
     bool m_display_link_notification_pending { false };
 };