Add wxFontEnumerator::InvalidateCache()

This is necessary to recognize new fonts, for example those added using
AddPrivateFont(), as being valid: otherwise they were not found in the
cache and couldn't be used in wxGTK.

src/common/fontenumcmn.cpp

@@ -127,6 +127,12 @@ bool wxFontEnumerator::IsValidFacename(const wxString &facename)
     return true;
 }
 
+/* static */
+void wxFontEnumerator::InvalidateCache()
+{
+    gs_allFacenames.clear();
+}
+
 #ifdef wxHAS_UTF8_FONTS
 bool wxFontEnumerator::EnumerateEncodingsUTF8(const wxString& facename)
 {