Fix the coordinates in wxDataViewCustomRenderer::LeftClick() in generic wxDVC.

Fix the coordinates in wxDataViewCustomRenderer::LeftClick() in generic wxDVC.

The code in generic wxDataViewCtrl implementation didn't take the space taken
by tree expander button nor the indentation of child nodes into account, so
the coordinates passed to LeftClick() could be completely wrong.

Fix this by offsetting them to ensure that click coordinates are always
relative to the cell client area.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68836 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775