Add timer to control the emit change target temp

Summary:
Add timer to control the emit change target temp

In every event on the thermo dial is emitted
to the printer the change of the target temps,
that can give us bad behaviors. So we are
using a QTimer with single shot to manage
those events.

Signed-off-by: Lays Rodrigues <lays.rodrigues@kde.org>

Reviewers: rizzitello, patrickelectric, tcanabrava

Reviewed By: rizzitello, tcanabrava

Differential Revision: https://phabricator.kde.org/D14831