Description

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

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

This depends on #631 and #636.
Until #631 and #636 are merged, it might be easier to view the changes made in this pull request by viewing the diff between this and #636 at https://github.com/BenjaminAmos/DestinationSol/compare/nui-inventory-screen...nui-game-menu.