"Pinned" nodes for layouts

[jacomyal]

There are currently two concurrent implementation plans:

1. Layout algorithm that should support that feature can have a new "Fixed node attribute" input, that only allows selecting a boolean attribute. Users will have to first create and fill that attribute as they want (which is not great), but at least the plan is easy, and fits well within existing Gephi Lite abstractions

2. We add a new technical attribute for nodes, named "Pinned" for instance, that has a lot of impact:

• Pinned nodes must be slightly different on the graph display (and the related data table display)
• There must be a new data table column to display and allow modifying this new attribute
• There must be easy controls to pin / unpin nodes
• We must decide what to do with pinned nodes for all layouts (including circular, random, etc...)