Treat the prompt line like a regular buffer

[lenormf]

Hi,

Following the implementation of #800, I figured treating the prompt line just like any other buffer would allow applying different sets of bindings without having to hardcode them (emacs/vim bindings for instance).

The current implementation doesn't allow things like the following:

map global prompt <c-u> Ghd

Cheers.

[greduan]

I think this is a good idea, just to make sure that it's in insert mode by default and that escape closes the prompt, since this is now expected behaviour.

[danr]

Additionally, this could open up for new completion possibilities in the prompt and possibly remove some duplicatied functionality. The current insertion mode completion could be used in this one-line-high prompt buffer, and the completion logic could be moved to kakrc scripts instead of -shell-completions and -shell-candidates etc.

[JJK96]

I am currently lookin into this one to see how much would need to be changed.

Currently I have that the InputHandler::Prompt class would be reimplemented.

Probably also something related to how it is displayed, but I cannot find where the Prompt is drawn/displayed. In ncurses_ui.cc I cannot find it. Maybe someone with more knowledge on the codebase can help me?