Usability improvements

I didn't expect so many people to play the game right away (thank you!), so here's a first patch that brings usability improvements. I haven't spent much time on the game's UI yet, and this should improve your experience a lot if you play the game without reading the itch page.

I'll work on savegames tomorrow.

Change notes

- Remove Stamina mechanic (I didn't like it anyway, and it was unclear the peasant was dying of exhaustion)

- Make the "Buy new track button" much more obvious. However, it doesn't show right from the start anymore, because it doesn't need to.

- Level-up buttons disable themselves when you don't have enough money

- There's now a hint that tells you t olevel up one of your peasant's skills when he's killed a few enemies.