[Views] Add Outline support

Papyrus/GMF provides an Outline view, which displays the entire diagram in a small view, along with the current editor viewport area. This is useful to quickly navigate large diagrams, and get a general overview:


This would be especially useful in GEFx, since the canvas has an infinite size, where it can be difficult to find elements that have been added far away from the center.

However, we must be careful about performances: the Papyrus/GMF Outline view introduces some severe performance issues when the viewport size changes (It needs to redraw the entire outline image, which can be very slow). On the other hand, incremental outline updates work fine on Papyrus/GMF.

Start by studying the Papyrus/GMF Outline behavior described in the issue and how the GEFx canvas represents its viewport. Define an outline that shows the full diagram and current viewport, then verify that updates are incremental rather than redrawing the entire outline on viewport changes. Done means large, widely spaced diagrams remain navigable without the severe performance problems noted.

Written by the indexing model from the issue text.