Replace color strings with Color.CONSTANTs

[samreid]

For instance, Panel.js has:

  var DEFAULT_OPTIONS = {
    fill: 'white',
    stroke: 'black',
    lineWidth: 1, // width of the background border

Using colors like that instantiates extra objects and takes more work, it seems it would be better to use Color.WHITE and Color.BLACK. I'm seeing many occurrences of 'white' and 'black' throughout the sim. @jonathanolson can you confirm that this would be a desirable change?

[jonathanolson]

It may be a minor memory/performance win to directly reference color objects like that. Probably not significant unless there are a large number of objects of a specific type.

[samreid]

Perhaps we should keep this in mind for sims that are known to have performance problems. Maybe I'll add a section to the developer guide?

[samreid]

The development overview is now sporting a performance section: #312