templater: move label() function from color extension
ui.label() is no-op by default, so we can just call ui.label() by label()
template function no matter if the color is enabled or not.
templater: make label() take unknown symbol as color literal
Instead of the mapping hack introduced by
b775a2029e8d, this patch changes the
way how a label symbol is evaluated. This is still hackish, but should be more
predictable in that it doesn't depend on the known color effects.
This change is intended to eliminate the reference to color._effects so that
color.templatelabel() can be merged with templater.label().