Add placeholder Screen classes

The PlayScreen is an abstract Screen that will provide default
functionality that is common between the number solver and letter solver
game screens. This functionality includes drawing and managing various
bits of UI such as the clock, scores, input boxes and some other elements
Add basic clock

There is a clock rendered on screen. It has a face, it WILL have a hand,
and it has a shade which covers the portion of the face that has already
been covered by the hand.

Currently the shade is opaque-- it should be transluscent.

There are no ticks on the clock face yet. There will be eventually.

To render the mesh, the spritebatch needs to be ended and begun in order
to have the occlusion work as intended. This means each frame will have
at least two render cycles if the clock is present, and that seems
pretty bad. Hopefully this can be fixed.
Clock is prettier, has a hand

There is now a pseudo-transluscent shade that covers parts of the clock
where the hand has swept past. In other news, there is a hand on the
clock which ticks around the face.

The clock hand icon is currently twice as tall as it needs to be,
because I can't figure out how to get a sprite to rotate around a point
that is not the center of the sprite. To be fixed, if I feel like it.

There is still an issue with rendering a mesh in transparent colors, so
currently the color has just been set manually to a "slightly darker
than the face" color. If the face ever gets tick marks, this could be a
problem.