Per-element layouts / selective layouts

[maxkfranz]

Allow layouts to be run on specified subsets of the graph.

Previously:
requires layout restriction flag to be per-element rather than per cy instance

[maxkfranz]

Issues:

• Layouts that would want to interact with elements outside the layout set
• Running 2 layouts at once whose elements sets intersect (per element locks solve this but will end up giving non desired behaviour to the user)

Just lock the elements you don't want the layout to affect and run the layout for now.

[maxkfranz]

Usecase for more complex behaviour: Adding and removing a node from a continuous layout
Limitation: Only one layout can "own" a node at a time

[maxkfranz]

Alternative: Rather than supporting multiple layouts running at once -- which has no known usecase -- use a mechanism for eliding elements from a layout?

Granularity for continuous/FD layouts? Included but not set vs not included

[maxkfranz]

Also refer to http://stackoverflow.com/questions/24519799/how-does-the-layout-function-work-on-collections-of-nodes