#151 "Modernise" keysyms (PgUp and PgDn)

[Arjen Markus]

I noticed that some common keys found on most modern
keyboards are not in the present list of keysyms or
only under an obbscure synonym.

Most noticeably these are: PgUp and PgDn.
They are present under the "old" names Prior and Next,
which to the average programmer used to PC keyboards
will not be obvious.