BDV visibility modes.

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To use them, press V in a BDV window.

4 modes are implemented:

• ALL: everything is visible.
• NONE: nothing is visible.
• FOCUS: the track of the focused vertex is visible.
• SELECTION: the current selection is visible.

The Visibility objects are used every-time one has to decide on the visibility of a vertex or an edge. This includes calls to methods like #getVertexAt() or #getEdgeAt(). Also the methods called by the BdvContextProvider, to keep visibility consistent.

The visibility itself is not super clever. We simply rely on the isVisible() method inside for loops over the vertices and edges returned by the clip convex polytope.

[tinevez]

Fixes #91 Fixes #96

[tinevez]

Integrated since beta-21