Fix hiding the 'say' bubble WRT number formatting

When setting bubble text, numbers are rounded to two decimal places if
necessary. The check before hiding the bubble that makes sure the
current bubble text matches the current value -- probably intended to
make sure it's "our" bubble -- now uses the same text formatting code to
avoid false negatives.
`source` really doesn't make sense in this `showBubble` since you always create a new `TalkBubble` and never check the source of the old one. Maybe you should just pass `null`?