Add some Screens that will be involved in selecting a level.

Level Select Screen and Level Status Screens have been added. I updated
the Main Menu class to transition to the Level Select Screen, the Level
Select Screen transitions to the Level Status Screen, and the Level
Status Screen transitions to the Gameplay Screen.
Updated the GameplayScreen.cs and the Game1.cs to work with a pause menu.

Added the pause menu back, so the player can only exit the game through
the main menu now.
Merge remote-tracking branch 'origin/master' into michael_new_menu

Conflicts:
	PuzzlePathDimension/Game1.cs
	PuzzlePathDimension/Menus/GameplayScreen.cs
Updated some Menus and Input Handlers for user

Update the Help Menu Screens and change input in order to match the
visual presentation of buttons.
Code cleanup

I removed some commented code that was not necessary and added some
comments to explain the different fields and methods.
Merge branch 'michael_new_menu'

Conflicts:
	PuzzlePathDimension/Menus/GameplayScreen.cs
Add Context and Pause inputs, and a Connected property.
Merge branch 'master' into x360-input
Remove the old functionality of the Back button.

Also, hard-code the X button to do a restart of the level.
Add IsButtonDown() and IsButtonUp().