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.

interface/wx/fontenum.h

@@ -73,6 +73,17 @@ public:
     */
     static bool IsValidFacename(const wxString& facename);
 
+    /**
+        Invalidate cache used by some of the methods of this class internally.
+
+        This method should be called if the list of the fonts available on the
+        system changes, for whatever reason. In particular, it is called
+        automatically by wxFont::AddPrivateFont().
+
+        @since 3.1.1
+     */
+    static void InvalidateCache();
+
     /**
         Called by EnumerateFacenames() for each match.