all: make unit.Converter concrete and rename to Metric

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>

io/system/system.go

@@ -17,10 +17,11 @@ import (
 // operations that describes what to display and how to handle
 // input.
 type FrameEvent struct {
-	Config Config
 	// Now is the current animation. Use Now instead of time.Now to
 	// synchronize animation and to avoid the time.Now call overhead.
 	Now time.Time
+	// Metric converts device independent dp and sp to device pixels.
+	Metric unit.Metric
 	// Size is the dimensions of the window.
 	Size image.Point
 	// Insets is the insets to apply.
@@ -51,12 +52,6 @@ type FrameEvent struct {
 	Queue event.Queue
 }
 
-// Config defines the essential properties of
-// the environment.
-type Config interface {
-	unit.Converter
-}
-
 // DestroyEvent is the last event sent through
 // a window event channel.
 type DestroyEvent struct {