IsEmpty() didn't exist in all ports (notably not wxMSW) and its meaning was
unclear anyhow, so remove it even from the ports where it did exist and add
clear Is{List,Text}Empty() replacements instead.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68808 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775