Add Tetris Model, View. Add L and T pieces.

Adds the Tetris model used for representing the Tetris game's current
state. Adds the View that draws the current state to the screen. Adds
two Tetris pieces (T and L). Renames Application to TetrisGame. Adds the
texture used for drawing the squares that make up each Tetris piece.
Revert "Add Tetris Model, View. Add L and T pieces."

This reverts commit e53c07cbade11269216f505bfc6930b65dc5f242.
Revert "Revert "Add Tetris Model, View. Add L and T pieces.""

This reverts commit 105691bfff61d0de8a575c5adcbe23e0c8d0795b.
Remove Application.cs