Implement the window_role criterion (checks WM_WINDOW_ROLE)

Closes: #446

This is handy for matching specific windows of a multi-window application, for
example only Pidgin’s buddy list window.

src/cmdparse.l

@@ -155,6 +155,7 @@ no                              { return TOK_DISABLE; }
 
 class                           { BEGIN(WANT_QSTRING); return TOK_CLASS; }
 instance                        { BEGIN(WANT_QSTRING); return TOK_INSTANCE; }
+window_role                     { BEGIN(WANT_QSTRING); return TOK_WINDOW_ROLE; }
 id                              { BEGIN(WANT_QSTRING); return TOK_ID; }
 con_id                          { BEGIN(WANT_QSTRING); return TOK_CON_ID; }
 con_mark                        { BEGIN(WANT_QSTRING); return TOK_MARK; }