Variables moved from player.properties to player

We used `player.properties` because that's where Tiled dumped
the custom variables we assigned to Player in the editor. This
was sort of silly and made code a lot more verbose than it needed
to be.

So things like `player.properties.speed` are now `player.speed`
and their values are set at the beginning of the program.
Added comments; removed useless code; added reset

Now pressing `r` moves the player back to its spawn point.