[WIP] start using new codemirror completion

Should start fixing as you type completion. 

This use the brand new completion-hint introduced by codemirror short after I wrote the completer.
it automatically handle the "as you type". 

The only annoying things I see and I'm not sure how to handle :  
   - We prevent event "downarrow" from propagating when the cursor is at the last char of codemirror, whic prevent the selection of next item in completer. 
   - esc dismiss completer and escape to command mode (but not always)