Fold best size and ellipsis handling in AutoResizeIfNecessary()

Refactor wxStaticText code to use AutoResizeIfNecessary() for all the
updates done after the label extent changes (whether it's due to the
change of the label itself or the font used for rendering it).

There should be no changes in behaviour after this commit except for a
small bug fix in wxGenericStaticText when setting label with markup:
this didn't invalidate the best size before, but does now.

src/generic/stattextg.cpp

@@ -102,10 +102,7 @@ void wxGenericStaticText::SetLabel(const wxString& label)
     wxControl::SetLabel(label);
     DoSetLabel(GetEllipsizedLabel());
 
-    InvalidateBestSize();
-
-    if ( !IsEllipsized() )
-        AutoResizeIfNecessary();
+    AutoResizeIfNecessary();
 
 #if wxUSE_MARKUP
     if ( m_markupText )