Don't handle "Return" key as "TAB" even when the default button is disabled.

Don't handle "Return" key as "TAB" even when the default button is disabled.

wxMSW used to handle VK_RETURN in the same way as VK_TAB if it wasn't consumed
by the default push button but this didn't correspond to the native platform
behaviour which considers pressing Return when the OK button is disabled an
error and audibly notifies the user about it.

Fix this by passing VK_RETURN to IsDialogMessage() if we don't translate it to
a button click.

Also add a possibility to test what happens when the default (or all)
button(s) in the dialog are disabled to the dialogs sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68227 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775