Rename wxStaticText::Do[SG]etLabel() to WX[SG]etVisibleLabel()

The names of these methods were confusing because they implied that they
were the actual implementations of the public [SG]etLabel(), while this
wasn't at all the case.

Give them then ames describing what they really do and also update the
comments to hopefully be more clear.

No real changes.

src/common/stattextcmn.cpp

@@ -240,9 +240,9 @@ void wxStaticTextBase::UpdateLabel()
     // In fact, we must be careful not to touch the original label passed to
     // SetLabel() otherwise GetLabel() will behave in a strange way to the user
     // (e.g. returning a "Ver...ing" instead of "Very long string") !
-    if (newlabel == DoGetLabel())
+    if (newlabel == WXGetVisibleLabel())
         return;
-    DoSetLabel(newlabel);
+    WXSetVisibleLabel(newlabel);
 }
 
 wxString wxStaticTextBase::GetEllipsizedLabel() const