improve auto scrolling label

[mattneub]

This PR tackles a number of issue connected with the auto scrolling label:

• There were some circumstances where the auto scrolling label would keep scrolling even though it was in the "background"; this PR stops the scrolling during those circumstances.
• The entire layout of the auto scrolling label was deeply troubled. The problem stemmed from three sources (and this PR makes revisions with respect to all of them):
  • Some instances of auto scrolling label, but not all, need to display their text centered if it isn't long enough to scroll.
  • Some instance of auto scrolling label, but not, need to be able to change their contents "live".
  • There was a misunderstanding about what a scroll view's content view is for and how to use it correctly.
• Due to a bug, the temporal gap between scrolls could get longer and longer as time went by. This PR fixes that.
• Some efficiencies are also introduced.

[einsteinx2]

Haha I had just mentioned in the other PR that there was already an auto scrolling label, but looks like you had already found it :)

This is fantastic, thank you! I'm going to also keep this PR open for now as I want to make sure to get these improvements to the new Swift port as well which still uses this class.