WIP : Feature completer

This is not finished yet but as I would like some feedback I open it now.
This adds completion capabilities through a new ToolkitObject, the Completer, which can be declared as a child of a field (I plan to add the MultilineField and the Scintilla widget in the future). The completion can be customized through three declarative function : 
- propose_completion called on each keystroke, allow the user to determine whether or not to complete and on which prefix, and to change the model, etc
- complete called when the user select an entry. It allows the user to customize how the replacing occurs (allow to add special char, etc)
- entry_hilighted to react to an entry being highlighted.

The docs are still missing for the most part but I will add them quickly.

Question : 
- as some stuff such as the child handling (which for now sucks) will be identical for Field, MultilineField, Scintilla is it worth creating a dedicated subclass ?