Positionable Widgets

cc @jmatsushita

>In order to introduce widgets for autocomplete, file browsing and so on, there seems to be a need to extend the widget functionality to render them elsewhere than at the top/bottom/left/right of a View.
> Allow Widgets rendering to be:
> - floating (overlaid maybe above/below/right/left of a specific position in text?)
> - block (inserted/embedded inside a text block)
> Seems that making the Widgets depend on a text position would allow to not leak the rendering abstractions. Does that work?

Start by examining the existing Widget and View abstractions to understand how rendering positions are currently represented. Determine how floating and block widgets should relate to text positions, then define completion around supporting both requested placement modes without leaking rendering abstractions.

Written by the indexing model from the issue text.