Autocomplete improvement

Enable custom regexprs to be setup for computing the prefix (same strategy than C9 where the indentifierRegexp of the first proposal  is taken).
This enables completers to expose the regexprs that I used to compute proposals.

I tried it on the XQuery mode for complex proposals (function calls with namespaces, namespaces URIs) and it works beautifully. With the LiveAutocomplete feature, it really provides an incredible editing experience.