Input manager cleanup

This is a collection of commits to slightly improve the input manager and to fix some oddities.

The space key is now represented by “SPACE”, not by the string “ ” (which looked odd).

Three menus have now a little help text which show now that “?” (or whatever `HELP_KEYBINDINGS` is) can be used: New character, construction, overmap. The player may not be aware of the possibility of using “?” otherwise.

A few default keybindings have been changed to remove some annoying collisions. See the commit for more info.

Lastly, the world manager and construction menu now show the actual keybindings instead of fixed ones.