Spending XP: when out of XP, needs to prevent spending more

[farfromunique]

Currently doesn't

[farfromunique]

Also: prevent selecting invalid dots

[farfromunique]

Turns out this will be a much larger undertaking -- dot click event listeners need to "uncheck" when they are clicked if there aren't enough points; this uncheck needs to not refund points.

alternate method: disable dots that are too expensive.