Rework level up dialog to behave more like vanilla

What this does exactly:
* Forbid assigning coins to maxed out attributes
* Gray out maxed out attributes (this wasn't in vanilla, but I think it's a nice touch) and don't show their multipliers
* Constant for maximum number of coins that can be assigned
* For instance, if there's only 1 attribute that is not maxed out, display just 1 coin instead of all 3
* Some code style consistency
Thanks for the tip, I'll keep that in mind.