Define and create access to a callback table

The internal table that stores a map between key values and callback
functions. Keys are abstracted with the type `RAVKEY` and are equal to
the values found in 'Include\ravk.h'. `RAKEYCALLBACK` is the type of the
callback function. The table is accessed though the stereotypical `Get*`
and `Set*` functions but attempts to access the callback to call the
function should be made through `CallKeyCallback`.

The callback table exists as link between the library and the
application making use of the library. Because Radium is not concerned
with the actions that need to be performed it makes no stipulation that
something _has_ to happen applications don't need to register callbacks
for every single function.

Currently, there is no public API feature available for applications to
register callbacks.