Add Pong for Nexys Video

[WillGreen]

Add Pong for Nexys Video and make several small improvements:

• xc7-hd hedgehog and hourglass designs use params for colour widths
• Make simple_score params explicit in top modules
• Put declaration of pix_addr before first use in 'simple_score.sv'
• Remove unused 'clock_tb.sv' sim module from Arty Pong
• Remove superfluous whitespace