Convert the in-game menu to NUI

[BenjaminAmos]

Description

This pull request converts the in-game menu to NUI. It should be functionally identical to before.

Before

After

Testing

• Start a new game or continue an existing one
• Open the in-game menu with Esc
• Pressing the Can sell used items button should toggle its value between Yes and No
• Pressing the Sound Volume and Music Volume buttons should change the sound volume and music volume buttons respectively
• Pressing the Respawn button should respawn the player ship
• Pressing the Exit button should exit the game to the main menu
• Pressing the Resume button should close the in-game menu
• Pressing the Esc key should be the same as pressing the Resume button

Notes

• This superseeds NUI game menu #638.
• This depends on New NUI widgets (EmptyIfInvisibleContainer, UIBackingBox and UILabelledIcon) #664 and Convert remaining MainGameScreen visuals to NUI #665.
• Until New NUI widgets (EmptyIfInvisibleContainer, UIBackingBox and UILabelledIcon) #664 and Convert remaining MainGameScreen visuals to NUI #665 are merged, it might be easier to view the changes made in this pull request by viewing the diff between this and Convert remaining MainGameScreen visuals to NUI #665 at BenjaminAmos/DestinationSol@nui-screen-conversion/main-game-screen...nui-screen-conversion/menu-screen.
• It may be difficult to test this without the fixes included in General NUI Fixes #663.