Add pause menu

[benfrankel]

Relates to #206.

NOTES:

• This PR only adds the menu without actually pausing gameplay.
• This PR doesn't add a translucent pause overlay.
• I hard-coded the "go back" feature, which would be better implemented with a state stack.
• I can't write "upon exiting any screen, close the current menu" using bevy_state because there is no state pattern-matching, so this is hard-coded as well. (well I can, but it wouldn't be simple enough for this template)
• There are probably 1-frame delays between entering a Screen and opening / closing the relevant Menu, because I can't do dependent states in bevy_state (aka require state transitions of Menu to run .after state transitions of Screen so that the latter can control the former on transition within the same frame).