Pause screen!

Before this, I had the pause button simply take the player to the options menu to adjust sound. This menu also (and I didn't realize this until right before fixing it) led the player back to the home screen instead of the game when they clicked back, unless they had paused the game via ESC. Whoops!

Updates:

• Added a pause menu, with the ability to open the options menu or the controls menu, as well as quit to the main menu.
• Adjusted the options and controls menu, once the game has been started, so that their "back" buttons go to the pause menu rather than the main menu, and the "back" button in the pause menu goes back to the game where it was.
• (Hopefully) fixed the Gamepad controls. This still needs to be tested.
• Added a 3d appearance for arrow buttons (all the other UI elements had both a 2d and a 3d already).

What's next:

• Test game with gamepad (or, if someone wants to try it out and leave a comment, that'd be amazing! I don't have a gamepad so otherwise I'll have to borrow my partner's).
• Add keyboard and gamepad support for adjusting volume in options menu (currently, both can navigate all other menus and use the button with enter or A/X, but volume needs to be adjusted with the mouse).
• Adjust the options and controls menu so that once someone has gone back to main menu, their "back" buttons go back to the main menu again.