get rid of most slowdown at notebook loading....
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
