LibGUI: Have widgets signal willingness to accept drops

If a widget accept()'s a "drag enter" event, that widget now becomes
the application-wide "pending drop" widget. That state is cleared if
the drag moves over another widget (or leaves the window entirely.)

Libraries/LibGUI/Widget.h

@@ -302,6 +302,8 @@ public:
     void set_shrink_to_fit(bool);
     bool is_shrink_to_fit() const { return m_shrink_to_fit; }
 
+    bool has_pending_drop() const;
+
 protected:
     Widget();