Consider "Saving...." notification.

Especially with slow backend, I found myself pressing save a few time as there is no notification that something is happening.

I would like to consider a sticky notification that appear when user ask to save and would be dismissed
after.

Code is [relatively simple](https://github.com/Carreau/ipython/commit/5636d56f53b269d30951adab961a8269f657b2a0) but Thomas think that it could be a anoyingly flashing thing and would prefer to "delay" the "Saving..." message by 200 something ms. Though, is saving take 260ms, you get a flash of 10ms... and whatever XXXms you delay the "Saving...." the saving could always finish a XXX+10ms leading to a flash...

Anyway, I want to propose that both in notebook and Editor.