Remove support for very old metacity/libwnck versions.

configure.ac:
	Require libwnck >= 2.19.4. Remove AC_CHECK_FUNCS for wnck_window_has_name.
	Require libmetacity-private >= 2.23.2

gtk/window-decorator/fusilli-decorator-gtk.c:
	Assume libwnck >= 2.19.4 and metacity >= 2.23.2
	show_force_quit_dialog:
		Make sure no NULL pointer is passed to g_markup_escape_text