Focus on the alert text when there is an error

This is to tell screen readers that have to read the text in the
notification, or the notification pops up and orca, at least, does not
know that it needs to read it.

I also had to change the role of SelectableLabel’s TextArea to
StaticText or orca would only read the first line. Not sure if that is
an error, because TextArea does indeed behave like a StaticText (i.e., a
label) in this case, but TextArea has Accessible.multiLine set to true.

src/ErrorNotification.qml

@@ -45,6 +45,8 @@ Control {
             id: label
 
             Accessible.ignored: !control.visible
+            Accessible.role: Accessible.StaticText
+            focus: control.visible
 
             anchors {
                 left: parent.left

src/SelectableLabel.qml

@@ -10,6 +10,9 @@ Control {
     contentItem: TextArea {
         id: textArea
 
+        Accessible.ignored: control.Accessible.ignored
+        Accessible.role: control.Accessible.role
+        focus: control.focus
         padding: 0
         readOnly: true
         selectByMouse: true