QEMU RFB extension - rfb.js and input.js changes

QEMU RFB extension - rfb.js and input.js changes

In input.js, a new keyboard handler was added to deal exclusively
with the QEMU key event extension. '_onKeyPress()' signature
was changed to allow the same method to treat both cases.

The extension will only be enabled if the browser has support
for the KeyboardEvent.code property.

Changes in rfb.js:

- added a new extension code, QEMUExtendedKeyEvent, value -258.

- handleKeyPress now receives 'keyevent' instead of 'keysym' and
'down'. Both values are retrieved from keyevent as they were
in the previous signature. This method now can send QEMU RFB
extended key messages if the flag was set to 'true'.

- tests/test.rfb.js were changed folowing the onKeyPress() signature
change.

- added a new function to send the QEMU extended key message.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>