This adds a function that converts an action (such as interacting
in-game) to the corresponding button text ("ENTER", "E") or button
glyph (PlayStation triangle, Steam Deck Y, etc). This function
currently only gives the existing ENTERs or Es, because I don't know
how best to detect controller usage, or whether the game is running on
a Steam Deck, or what buttons need to be displayed there. Still, it
should now be really easy to adapt the rendering of keyboard keys to
consoles, controllers, or rebound keys.
To identify the actions that currently need to be displayed, this
commit also adds the initial enums for action sets as described by
Ethan in a comment in #834 (Jan 18, 2022).
