WindowServer: Give a thread boost to the currently active window

When the currently active (foreground) window is owned by a client,
we now apply a +10 priority boost to the client's main thread.

You normally want the window you're interacting with to be responsive,
so this little boost allows it to run a bit sooner and more often. :^)

Servers/WindowServer/WSClientConnection.h

@@ -22,6 +22,9 @@ public:
     ~WSClientConnection() override;
     virtual void die() override;
 
+    void boost();
+    void deboost();
+
     static WSClientConnection* from_client_id(int client_id);
     static void for_each_client(Function<void(WSClientConnection&)>);