LibGUI: Prevent multiple drag initiations while drag messages are passed

Author: https://github.com/alimpfard
Commit: a2cfb7eb94
Pull-request: https://github.com/SerenityOS/serenity/pull/3984
Issue: https://github.com/SerenityOS/serenity/issues/3906
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/AbstractView.h

@@ -186,6 +186,7 @@ private:
     bool m_activates_on_selection { false };
     bool m_multi_select { true };
     bool m_tab_key_navigation_enabled { false };
+    bool m_is_dragging { false };
 };
 
 }