Improve lookup of special keys

Look up keys that are independent of layout and state first,
followed by keys that are only mild variations in layouts.
This is more robust as there might be multiple physical keys
generating the same symbols, and Keysyms don't map directly to
Unicode in all cases.

At the same time switch over to using the modern, standardised
'code' field for lookup.

tests/vnc_perf.html

@@ -51,7 +51,7 @@
                 'core': ["base64.js", "websock.js", "des.js", "input/keysym.js",
                          "input/keysymdef.js", "input/xtscancodes.js", "input/util.js",
                          "input/devices.js", "display.js", "rfb.js", "inflator.js",
-                         "input/vkeys.js"],
+                         "input/vkeys.js", "input/fixedkeys.js"],
                 'tests': ["playback.js"],
                 'recordings': [fname]});
         } else {