ui,ui/app: convert Config to an interface

To keep the interface slim, remove the helper methods and shorten
the essential method, Pixels, to Px.

Add and use unexported Config implementation in the app package.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/layout/layout.go

@@ -96,14 +96,14 @@ type Insets struct {
 	cs                       Constraints
 }
 
-func (in *Insets) Begin(c *ui.Config, ops *ui.Ops, cs Constraints) Constraints {
+func (in *Insets) Begin(c ui.Config, ops *ui.Ops, cs Constraints) Constraints {
 	if in.begun {
 		panic("must End before Begin")
 	}
-	in.top = c.Pixels(in.Top)
-	in.right = c.Pixels(in.Right)
-	in.bottom = c.Pixels(in.Bottom)
-	in.left = c.Pixels(in.Left)
+	in.top = c.Px(in.Top)
+	in.right = c.Px(in.Right)
+	in.bottom = c.Px(in.Bottom)
+	in.left = c.Px(in.Left)
 	in.begun = true
 	in.ops = ops
 	in.cs = cs

ui/layout/list.go

@@ -18,7 +18,7 @@ type scrollChild struct {
 }
 
 type List struct {
-	Config             *ui.Config
+	Config             ui.Config
 	Inputs             input.Events
 	Axis               Axis
 	Invert             bool