Selection is not preserved across new views

[ghutchis]

Selections are (mostly) stored in the GLWidget view. Selection of residues is not always updated properly, particularly across new views.

The selection code should uniformly use the GLWidget selection property and eliminate the selection information in the Primitive classes.

Reported by: @ghutchis

[ghutchis]

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Please see commit 387. I think this is an improvement but it might not be all the way there yet. Let me know what you think to it. Not sure if the residue selection might need more work.

Original comment by: @cryos

[ghutchis]

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This is fixed. All code now uses the GLWidget selection and the PrimitiveList class.

Original comment by: @ghutchis