applets/notifications: allow screen reader to read notification

This enables Orca to read notification summary and body.

Type	Accessible.name	Accessible.description
Popup	{Summary}	{Body} from {Application Name}

See also https://codereview.qt-project.org/c/qt/qtdeclarative/+/441777

Requires qt/qt/qtdeclarative!30 (merged)

Ref: plasma-desktop#41 (closed)