Add wxDataViewModel::ChangeValue() and use it in wxDVC implementation.

Add wxDataViewModel::ChangeValue() and use it in wxDVC implementation.

ChangeValue() is a trivial wrapper calling both SetValue() and ValueChanged().
It allows to replace many calls to SetValue() immediately followed by
ValueChanged() with a single function call which is significantly shorter and
less error-prone (e.g. most of the existing code didn't test SetValue() return
code at all).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62489 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775