Minor change to code completion but major improvement, especially for python:

 - fix column number: always request a completion at the beginning of the prefix so that we get the complete list of possible completions out of jedi, not just the one that match the current text. That way it is possible to use left key to adjust the prefix and discover even more completions.

 - allow completions request when using backspace or delete and the popup is not already visible.
Merged 'develop'.