Allow the visualizer to enable fog-of-war on the fly

[wschella]

Let people decide from which viewpoint the match should be displayed.
Options are:

• Player 1, 2, 3, etc individually
• A collective of players (no forts that both players can't see)
• Total view (currently used)

TODO:

• Add view picking buttons/dropdowns/...
• Add individual view
• Add collective view
• Add total view

[ninewise]

I'd place the "invisible" nodes in a lighter/more grayish colour. Then you can stil draw the same graph, yet it's clear what can be seen and what can't.