Ace code editor

[xavArtley]

This PR add a panel of type code editor
It's based on this library https://github.com/ajaxorg/ace/

I have activated the language and theme selection
basic autocompletion with ctrl+space
and possibility to add anotations

[philippjfr]

@xavArtley I'd be happy to merge this once there is a reference gallery entry. At some point we will have to have a discussion which components make sense in panel itself and which could be added somewhere else.

[xavArtley]

Indeed I think this model should be external to panel more in some custom extensions

[philippjfr]

We'll need to work out a plan for managing external components like this.

[philippjfr]

I'm actually happy to merge this now and move it later if we decide on a repo to extend panel. Does that sound okay @xavArtley ?

[xavArtley]

Ok for me