widget: replace newline with space in single-line Editors; cover SetText

Move the replacing to Editor.prepend to fix SetText, and replace with
space instead of nothing to keep lines separated even in single-line
Editors.

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

widget/editor.go

@@ -645,14 +645,14 @@ func (e *Editor) Insert(s string) {
 }
 
 func (e *Editor) append(s string) {
-	if e.SingleLine {
-		s = strings.ReplaceAll(s, "\n", "")
-	}
 	e.prepend(s)
 	e.rr.caret += len(s)
 }
 
 func (e *Editor) prepend(s string) {
+	if e.SingleLine {
+		s = strings.ReplaceAll(s, "\n", " ")
+	}
 	e.rr.prepend(s)
 	e.caret.xoff = 0
 	e.invalidate()