ktextedit: lazy load the QTextToSpeech object

Summary:
Create the QTextToSpeech object on demand, i.e. only at the first
request to speak a text. While this will make the the first request
slower (but only the first), the creation of each KTextEdit widget is
faster again.

Test Plan:
Builds fine, unit tests runs fine, ktextedittest still works as
expected.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16001