Fix handling of help buttons with non-empty label under OS X.

Fix handling of help buttons with non-empty label under OS X.

Creating a button with wxID_HELP and a non-empty label used to always show
this label, even if it was just a standard "Help" or "&Help". This didn't work
at all as the help buttons under OS X are too small to show any label.

To fix this, use normal buttons, not help ones, if a really custom label is
used to ensure that it can be seen. And to still use the correct help buttons
in as many cases as possible, ignore the standard "Help" label and its
variants and don't show it at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65258 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775