Camera follow player + Enemy/Cursor Textures + Better firing direction

[pserwylo]

Was going to use the MonoGame.Extended.Tween library to do nice smooth zooming/panning, but that turned out to be more targetted at animations where you knew the start position and the end position of something. When zooming/panning, the desired location/zoom is constantly changing every frame, so it worked much better to just use the MovementVelocity helper class you created for panning, and some manual interpolation for zooming.

[pserwylo]

Created a turret class and moved turret related code from the tank into this. Helps with things like:

• Determining what type of bullet to fire.
• Managing the bullet textures (instead of loading them in the top level Game class.
• Calculating exactly where the tip of the turret is.

In general, will make it easier to do things like implement different types of guns in the future.