Replace drawText with add and remove methods.

Every cache element now contains the actual text element instead of just
its HTML, plus a flag indicating whether it is visible.  The addText and
removeText methods control the state of this flag, and the render method
uses it to manage elements within the text container.  So where we
previously used drawText to actually render text, now we add each string
once, then let the render method take care of drawing them as necessary.

This dramatically improves performance by eliminating the need to clear
and re-populate HTML text on every drawing cycle.  Since the elements
are now static between add/remove calls, this also allows users to add
interactivity, as they could in 0.7.  Finally, it eliminates the need
for a separate 'hot' cache.

I also removed the unnecessary 'dimensions' object; it's easier and
faster to store the width and height at the top level of the info
object.