LibGUI: Preserve existing GItemView selection on rubber band (#1031)

Author: https://github.com/DAlperin 🔰
Commit: dcc4704fb5
Pull-request: https://github.com/SerenityOS/serenity/pull/1031
Reviewed-by: https://github.com/awesomekling

Libraries/LibGUI/GItemView.h

@@ -55,4 +55,5 @@ private:
     bool m_rubber_banding { false };
     Point m_rubber_band_origin;
     Point m_rubber_band_current;
+    Vector<GModelIndex> m_rubber_band_remembered_selection;
 };