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/GEvent.h

@@ -2,9 +2,9 @@
 
 #include <Kernel/KeyCode.h>
 #include <LibCore/CEvent.h>
-#include <LibGUI/GWindowType.h>
 #include <LibDraw/Point.h>
 #include <LibDraw/Rect.h>
+#include <LibGUI/GWindowType.h>
 
 class CObject;
 
@@ -34,6 +34,7 @@ public:
         WindowCloseRequest,
         ContextMenu,
         EnabledChange,
+        Drop,
 
         __Begin_WM_Events,
         WM_WindowRemoved,
@@ -278,3 +279,20 @@ private:
     unsigned m_modifiers { 0 };
     int m_wheel_delta { 0 };
 };
+
+class GDropEvent final : public GEvent {
+public:
+    GDropEvent(const Point& position, const String& text)
+        : GEvent(GEvent::Drop)
+        , m_position(position)
+        , m_text(text)
+    {
+    }
+
+    const Point& position() const { return m_position; }
+    const String& text() const { return m_text; }
+
+private:
+    Point m_position;
+    String m_text;
+};