ui/layout: rename and sanitize enums

Rename MainAxisAlignment to Spacing and CrossAxisAlignment to just
Alignment.

Drop the untyped Start, End, Center values and add them as Spacing
and Direction values. Center is both a Direction and Alignment, so
use the synonym "Middle" for the alignment.

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

ui/layout/list.go

@@ -8,8 +8,8 @@ import (
 	"gioui.org/ui"
 	"gioui.org/ui/gesture"
 	"gioui.org/ui/input"
-	"gioui.org/ui/pointer"
 	"gioui.org/ui/paint"
+	"gioui.org/ui/pointer"
 )
 
 type scrollChild struct {
@@ -18,9 +18,9 @@ type scrollChild struct {
 }
 
 type List struct {
-	Axis               Axis
-	Invert             bool
-	CrossAxisAlignment CrossAxisAlignment
+	Axis      Axis
+	Invert    bool
+	Alignment Alignment
 
 	// The distance scrolled since last call to Init.
 	Distance int
@@ -204,10 +204,10 @@ func (l *List) Layout() Dimens {
 	for _, child := range l.children {
 		sz := child.size
 		var cross int
-		switch l.CrossAxisAlignment {
+		switch l.Alignment {
 		case End:
 			cross = maxCross - axisCross(l.Axis, sz)
-		case Center:
+		case Middle:
 			cross = (maxCross - axisCross(l.Axis, sz)) / 2
 		}
 		childSize := axisMain(l.Axis, sz)