Fix QTime function deprecation warnings by switching to QElapsedTimer

Summary:
The following functions have deprecation warnings: QTime::start(), QTime::restart(), QTime::elapsed() and recommend that QElapsedTimer be used instead of QTime.
_clock.isNull() had to be changed to !_clock.isValid() since QElapsedTimer does not have isNull().
QTime::isNull() is also invalid, so behavior shouldn't change.

Reviewers: Breeze, Plasma, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: hpereiradacosta, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26268