RFE: add labels (3D) to color/transparency command targets

[Elaine Meng]

Consider adding a (3D) labels target to the color and transparency commands. I’m neutral as to whether labels should be in the default target, or whether when one specifies a label-model number directly, ChimeraX should know that labels are the target.

[Tom Goddard]

I agree this is worth adding. I expect it to be a while before getting to this.

[Elaine Meng]

Note "color" already includes labels in the default target (i.e. if no target specified, labels get colored too). The "color" command just doesn't allow limiting coloring to labels unless a label model number is specified. I.e. "color orange" will color everything orange, including labels, and "color #1.2 green" will make the label model #1.2 green. So the message to the Log "Label colors not supported yet" from commands like "color pink labels" or "color pink targ l" is misleading. (Obviously, the "label" command can color labels as well.)

[Elaine Meng]

The "transparency" command accepts "labels" or "target l" without complaint, but it doesn't change their transparency even if a label model number is specified.

Changing label color with "color #1.2 green" or "label color red" does not update the color well shown in the Models list.

Maybe these are separate bugs.

[Greg Couch]

The bugs are related.  The code for parsing the targets and what 
arguments was unified.

[Elaine Meng]

"color modify" does not act on labels although simple coloring does. E.g. if label model is #1.2, "color modify hue 10" and "color modify #1.2 hue 10" do not change the label color.