An interface for scaling dp and sp is overkill, at least for all
current uses. Make it a concrete struct type, and rename it to the
shorter and more precise Metric.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
Package ui is now only about units except for the Config.Now method.
Remove Now and rename Config to Converter. Add layout.Config to
replace the old ui.Config.
Signed-off-by: Elias Naur <mail@eliasnaur.com>