Improve menu nav and pause ux

[nathanleiby]

A handful of bugfixes and UX improvements:

• https://trello.com/c/YygyhVQC/43-cleverfall-unable-to-interact-with-pause-menu-during-combat
• https://trello.com/c/E2ThJQPc/53-cleverfall-bug-pause-game-before-entering-round-end-screen
• https://trello.com/c/fHfwF3Tj/54-allow-exiting-end-of-round-screen-via-next-button

Big changes are:

• mouse no longer can be used, to avoid accidental menu (un)select issues... we expect to play with gamepad, though keyboard is still supported.
• PauseMenu and EndCombatUI get instantiated on demand, rather than existing and show/hide via SetActive(bool). This works better with the EventSystem, which is now able to select a default button reliably.

See commits for other changes.

[nathanleiby]

Beyond UX improvements, this should make it possible for us to playtest better today. It's now possible to play multiple combat rounds without quitting and restarting the executable :D

[nathanleiby]

[nathanleiby]

Gonna merge so this goes into our gameday build. Happy to respond to feedback post hoc