add keysyms as needed

[Antoine Martin]

Following up #856: when the keymap doesn't have the keysym we need... we just can't handle the key event.
See also #1049

Solutions:

• pre-define more keysyms - doable, should already be the case - mostly
• add them on the fly?

With the HTML5 client, we don't really have a list of keysyms pre-defined in advance: the client is free to use whatever input method is available to it, and we get unicode characters in the events.