Add support for sorting by more than one column to generic wxDataViewCtrl.

Maintain a list of columns used for sorting instead of a single sort column
index and allow to add/remove columns to/from it interactively by right
clicking them if AllowMultiColumnSort() was used.

See https://github.com/wxWidgets/wxWidgets/pull/3

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75806 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -26,6 +26,7 @@ All:
 All (GUI):
 
 - XRC handler for wxAuiToolBar added (Kinaou Hervé).
+- Add support for sorting wxDataViewCtrl by multiple columns (Trigve).
 - Add wxHtmlWindow::SetDefaultHTMLCursor() (Jeff A. Marr).
 - Add default ctor and Create() to wxContextHelpButton (Hanmac).
 - Send events when toggling wxPropertyGrid nodes from keyboard (Armel Asselin).