Make SceneManager robustly handle Main Menu and Game swapping

[DevMichaelV]

The SceneManager needs to handle when the user presses ESC to open the Main Menu for going back to the GameSelection or quitting the game.

For running sub-games as standalone scenes while developing them:
As SceneManager is a Global in the project, it can check if the GameRoot node exists, and:

• Use the GameRoot to quit to the GameSelection, or
• Quit the game entirely

The Main Menu may consist of things like:

• Return to Game Selection
• Settings
• Exit Game

Notes / Caveats:
Some checking may need to be implemented, as games may have their own menus.
Developing a structured format for sub-game developers to adhere to for this project seems necessary.