get rid of most slowdown at notebook loading. 1) Do not setOption('mode',new_mode) on CM if new and old mode are the same. It triggert **a lot** of calculation of bounding box in the end. 2) Do **not** select cell when loading the notebook it triggers **a lot** of CM even that check visible things and so on and so forth. So add a option to add_cell_at_index not to select it 3) jQuery $.attr has some magics, but has a slight overhead on real native ELEM.setAttribute DOM method. Seem slight improvement when loads of PNGs on one page