The Volume is now in the common folder.

All includes have been changed.
The Label documentation now provides a bit of insight.

It is why the Label can't use a similar interface the Ball or Paddle.
src/client/render.h now includes common/Volume.h.

It was including server/Volume.h which is wrong.
The Button now has a configurable background color.

It can be configured separately for when it is enabled or disabled. Also
the MenuGameState doesn't specify a background color, so the menu is
ridiculously funny!
Fixed some broken logic in the Button class.

- The background color
- The text height

Both the problems were with properly notifying the label at the right
time.
Specified the now-unspecified values for buttons in the MenuGameState.

That would be background and text color. Since they are now configurable
they didn't actually have a default. In fact, they aren't even
initialized if they aren't set resulting in hilarious consequences.
Cleaned up the Button class to be better.

The code is just cleaner and the constructor has more arguments.