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/llui/lllineeditor.cpp

@@ -858,8 +858,7 @@ BOOL LLLineEditor::handleHover(S32 x, S32 y, MASK mask)
 		if (mScrollTimer.hasExpired())
 		{
 			S32 increment = llround(mScrollTimer.getElapsedTimeF32() / AUTO_SCROLL_TIME);
-			mScrollTimer.reset();
-			mScrollTimer.setTimerExpirySec(AUTO_SCROLL_TIME);
+			mScrollTimer.reset(AUTO_SCROLL_TIME);
 			if( (x < mMinHPixels) && (mScrollHPos > 0 ) )
 			{
 				// Scroll to the left