Rename display-params to window-params

In the course of planning the upcoming display implementation, I began
to look at the data being shared between what will eventually become my
system of threads.

As this is relevant to the display code, I noticed that my "Display
Parameters" actually fall more into two separate categories...the
data being displayed (the board, cursor position, time information etc)
and the parameters that determine how that information is shown in the
window (dimensions, colors, etc).

To reflect this and avoid confusion with a struct I'll add in a
future commit, I renamed this second class of parameters as
"window-params".