Fix wxStyledTextCtrl::WriteText() to replace the selection.

Fix wxStyledTextCtrl::WriteText() to replace the selection.

WriteText() must replace the selection, not just insert the new text,
otherwise SetValue() implementation inherited from the base class doesn't work
as it doesn't clear the old contents of the control before adding new text to
it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72594 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775