LibGUI+WindowServer: Start fleshing out drag&drop functionality

This patch enables basic drag&drop between applications.
You initiate a drag by creating a GDragOperation object and calling
exec() on it. This creates a nested event loop in the calling program
that only returns once the drag operation has ended.

On the receiving side, you get a call to GWidget::drop_event() with
a GDropEvent containing information about the dropped data.

The only data passed right now is a piece of text that's also used
to visually indicate that a drag is happening (by showing the text in
a little box that follows the mouse cursor around.)

There are things to fix here, but we're off to a nice start. :^)

Libraries/LibGUI/GDragOperation.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <LibCore/CEventLoop.h>
+#include <LibCore/CObject.h>
+
+class GWindowServerConnection;
+
+class GDragOperation : public CObject {
+    C_OBJECT(GDragOperation)
+public:
+    enum class Outcome {
+        None,
+        Accepted,
+        Cancelled,
+    };
+
+    virtual ~GDragOperation() override;
+
+    void set_text(const String& text) { m_text = text; }
+
+    Outcome exec();
+    Outcome outcome() const { return m_outcome; }
+
+    static void notify_accepted(Badge<GWindowServerConnection>);
+    static void notify_cancelled(Badge<GWindowServerConnection>);
+
+protected:
+    explicit GDragOperation(CObject* parent = nullptr);
+
+private:
+    void done(Outcome);
+
+    OwnPtr<CEventLoop> m_event_loop;
+    Outcome m_outcome { Outcome::None };
+    String m_text;
+};