Use client size when ellipsizing wxStaticText label

The full size includes the borders and the return value of this function
might not actually be fully visible if we use it, as it can fit into the
full width but not the client width. And at least under MSW this
resulted in the ellipsized label being wrapped, which was completely
unexpected, especially if the second line of the control was not visible
at all as it seemed that the last word simply disappeared.

Closes #18573.

src/common/stattextcmn.cpp

@@ -261,7 +261,7 @@ wxString wxStaticTextBase::GetEllipsizedLabel() const
 
 wxString wxStaticTextBase::Ellipsize(const wxString& label) const
 {
-    wxSize sz(GetSize());
+    wxSize sz(GetClientSize());
     if (sz.GetWidth() < 2 || sz.GetHeight() < 2)
     {
         // the size of this window is not valid (yet)