Make "Close" button affirmative ID in standard dialog button sizer.

This allows to use the Escape key to close a dialog with the "Close" button
only.

Closes #16185.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76421 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775