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/llpreviewtexture.cpp

@@ -446,8 +446,7 @@ void LLPreviewTexture::onFileLoadedForSave(BOOL success,
 		}
 		else
 		{
-			self->mSavedFileTimer.reset();
-			self->mSavedFileTimer.setTimerExpirySec( SECONDS_TO_SHOW_FILE_SAVED_MSG );
+			self->mSavedFileTimer.reset(SECONDS_TO_SHOW_FILE_SAVED_MSG);
 		}
 
 		self->mSaveFileName.clear();