Replace wxComboBox::IsEmpty() with Is{List,Text}Empty().

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

include/wx/univ/combobox.h

@@ -129,7 +129,9 @@ public:
         wxItemContainer::Clear();
     }
 
-    bool IsEmpty() const { return wxItemContainer::IsEmpty(); }
+    // See wxComboBoxBase discussion of IsEmpty().
+    bool IsListEmpty() const { return wxItemContainer::IsEmpty(); }
+    bool IsTextEmpty() const { return wxTextEntry::IsEmpty(); }
 
     // wxControlWithItems methods
     virtual void DoClear();