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/llcommon/llapp.cpp

@@ -249,15 +249,6 @@ LLSD LLApp::getOptionData(OptionPriority level)
 	return mOptions[level];
 }
 
-void LLApp::stepFrame()
-{
-	LLFrameTimer::updateFrameTime();
-	LLFrameTimer::updateFrameCount();
-	LLEventTimer::updateClass();
-	mRunner.run();
-}
-
-
 void LLApp::setupErrorHandling()
 {
 	// Error handling is done by starting up an error handling thread, which just sleeps and