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.cpp

@@ -0,0 +1,53 @@
+#include <LibGUI/GDragOperation.h>
+#include <LibGUI/GWindowServerConnection.h>
+
+static GDragOperation* s_current_drag_operation;
+
+GDragOperation::GDragOperation(CObject* parent)
+    : CObject(parent)
+{
+}
+
+GDragOperation::~GDragOperation()
+{
+}
+
+GDragOperation::Outcome GDragOperation::exec()
+{
+    ASSERT(!s_current_drag_operation);
+    ASSERT(!m_event_loop);
+
+    auto response = GWindowServerConnection::the().send_sync<WindowServer::StartDrag>(m_text, -1, Size());
+    if (!response->started()) {
+        m_outcome = Outcome::Cancelled;
+        return m_outcome;
+    }
+
+    s_current_drag_operation = this;
+    m_event_loop = make<CEventLoop>();
+    auto result = m_event_loop->exec();
+    m_event_loop = nullptr;
+    dbgprintf("%s: event loop returned with result %d\n", class_name(), result);
+    remove_from_parent();
+    s_current_drag_operation = nullptr;
+    return m_outcome;
+}
+
+void GDragOperation::done(Outcome outcome)
+{
+    ASSERT(m_outcome == Outcome::None);
+    m_outcome = outcome;
+    m_event_loop->quit(0);
+}
+
+void GDragOperation::notify_accepted(Badge<GWindowServerConnection>)
+{
+    ASSERT(s_current_drag_operation);
+    s_current_drag_operation->done(Outcome::Accepted);
+}
+
+void GDragOperation::notify_cancelled(Badge<GWindowServerConnection>)
+{
+    ASSERT(s_current_drag_operation);
+    s_current_drag_operation->done(Outcome::Cancelled);
+}