Make wxDataViewModel::GetAttr() and GetAttrByRow() const.

This is an incompatible change but having to use a non-const model pointer to
call a clearly logically const version was simply too ugly so change it while
we still can.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62500 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -390,6 +390,10 @@ INCOMPATIBLE CHANGE SINCE 2.9.0
   2.9.0. Please use UseAppInfo(AppInfo_AppName | AppInfo_VendorName) explicitly
   to use the vendor name in the paths returned by wxStandardPaths.
 
+- wxDataViewModel::GetAttr() is now const, as it should have been from the very
+  beginning. You will need to change it to be const in your derived model
+  class too if you override it.
+
 
 All: