Fix initial state of radio toolbar tools in wxGTK.

Fix initial state of radio toolbar tools in wxGTK.

The last radio group item was initially checked under wxGTK instead of the
first one, as under the other platforms. Fix this by correcting the check for
the first radio group item which was inversed in wxToolBar::DoInsertTool().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70510 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775