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/stack.go

@@ -25,19 +25,6 @@ type StackChild struct {
 	dims  Dimens
 }
 
-type Direction uint8
-
-const (
-	NW Direction = iota
-	N
-	NE
-	E
-	SE
-	S
-	SW
-	W
-)
-
 func (s *Stack) Init(ops *ui.Ops, cs Constraints) *Stack {
 	s.ops = ops
 	s.cs = cs