port filled rings from Chimera

[Greg Couch]

Filled sugar and bases rings has been requested as the default for all atom nucleotide representation. See #1157 for some background info.

[Greg Couch]

Chimera Ring API

• Per-molecule: aromatic ring display, aromatic ring style (circle, disk), cached aromatic ring info, cached filled ring info, dirty bit for ring graphics, hasRingInfo() that is true if any residue has filled rings
• Per-residue: filled ring display, filled ring style (thin, thick), filled ring color (a single color, or "None" for per-atom wedges)
• commands: aromatic [off | circle | disk], fillring [ off | unfilled | thin | thick], "color ,f" changes the ring fill color

It was done as part of the base molecule support to simplify the dirty logic (atoms move, change color, coordset changes)

ChimeraX Ring API

Also, integrate into core molecular code. Leave out aromatic rings for now. No per-atom colored rings. Residues will have a ring color, like they have a ribbon color, that defaults to the molecule color. Commands are:

rings atom-spec [fill | clear] [ thin | thick ]

Add rings target type to color command.

[Greg Couch]

Ported. For now commands are:

show rings thinRings false
show target f thinRings false
hide rings
color yellow rings
color yellow target f