static message instead of function call to prevent shell slowdown

[gerrit8143]

#11496

[Carreau]

Hum, unlike what I said in #11496 his actually breaks the prompt when using vi mode for example.
Vi mode ($ ipython --TerminalInteractiveShell.editing_mode=vi) shows in the prompt wether we are in insert or normal mode.

So we'll have to a be a bit smarter as to wether we compute this once, or regularly.

[Carreau]

I've push some changes on your brnach that shoudl fix the high number of call with the default mode, but keep the changing prompt when using vi keybinding. Let me know what you think.

[gerrit8143]

looks good to me, so let's do it that way.

[Carreau]

looks good to me, so let's do it that way.

Thanks, I'll update the whats new note when I have time and then merge.
Should be released by end of month.