LLFrameTimer review and resulting changes.

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).

indra/newview/llappviewer.cpp

@@ -3501,8 +3501,7 @@ void LLAppViewer::idle()
 	// Update frame timers
 	static LLTimer idle_timer;
 
-	LLFrameTimer::updateFrameTime();
-	LLFrameTimer::updateFrameCount();
+	LLFrameTimer::updateFrameTimeAndCount();
 	LLEventTimer::updateClass();
 	LLCriticalDamp::updateInterpolants();
 	LLMortician::updateClass();