added properties to screen

now screen.w and screen.h give back size
and screen.initialized says if init() was succesfull
plus the array layers[] contains the list of currently added layers

scripts/javascript/examples/vector_test.js

@@ -1,7 +1,7 @@
 // simple test for CairoLayer
 
 scr = new Screen();
-if (!scr.is_initialized()) {
+if (!scr.initialized) {
     // screen hasn't been initialized yet, let's do it now
     scr.init(400,300);
     add_screen(scr);