This makes the class API a bit more sane, although
only a bit, because I had to leave it working with
possibly new code merged in from LL: the API can't
really change. I also removed some unused code.
While reviewing how LLFrameTimer worked however,
I did find a few instances where things where broken:
* sFrameDeltaTime wasn't correctly updated (more
than once per frame and therefore erratic). This
only influenced scrolling speed, but still.
* While dragging an inventory item, the scroll
arrows of a tab container didn't work
(LLTabContainer::handleDragAndDrop).
* Map zoom interpolation was broken (it interpolated
between A and B, but used the already updated
interpolation for A the next frame...
(added mCurZoomValInterpolationStart).
