Positionable Widgets

[ChrisPenner]

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?