Feature: Add CListCtrlEx with column show/hide support

- Create CListCtrlEx class derived from CListCtrl
- Support right-click header menu to toggle column visibility
- Save column visibility settings to registry (list\{ConfigKey})
- Settings persist by column name, not index (order-independent)
- Skip empty column titles in context menu
- Update CClientListDlg to use CListCtrlEx
- Update m_CList_Online in 2015RemoteDlg to use CListCtrlEx

server/2015Remote/2015Remote_vs2015.vcxproj

@@ -287,6 +287,7 @@
     <ClInclude Include="CDrawingBoard.h" />
     <ClInclude Include="CGridDialog.h" />
     <ClInclude Include="Chat.h" />
+    <ClInclude Include="CListCtrlEx.h" />
     <ClInclude Include="context.h" />
     <ClInclude Include="CPasswordDlg.h" />
     <ClInclude Include="CRcEditDlg.h" />
@@ -382,6 +383,7 @@
     <ClCompile Include="CDrawingBoard.cpp" />
     <ClCompile Include="CGridDialog.cpp" />
     <ClCompile Include="Chat.cpp" />
+    <ClCompile Include="CListCtrlEx.cpp" />
     <ClCompile Include="CPasswordDlg.cpp" />
     <ClCompile Include="CRcEditDlg.cpp" />
     <ClCompile Include="CTextDlg.cpp" />