Fix selection events generation in multi-select wxTreeCtrl.

Only deselect the items when the mouse is released if really necessary: add a
flag indicating when it is instead of trying to deduce it in the mouse up
handler.

Closes #11099 (thanks to Jonathan Liu).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61784 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/treectrl.h

@@ -318,6 +318,9 @@ private:
     // whether we need to trigger a state image click event
     bool m_triggerStateImageClick;
 
+    // whether we need to deselect other items on mouse up
+    bool m_mouseUpDeselect;
+
     friend class wxTreeItemIndirectData;
     friend class wxTreeSortHelper;