Convert MythTimer to use QElapsedTimer instead of QTime.

Fixes #11255.

We initially created MythTimer because QTime isn't monotonic
and we wanted a wrapper that assured that time wouldn't march
backward too much. As of Qt 4.7 there is a timer available
which is monotonic on most systems. This converts MythTimer
to use this new timer.

The first version of this new wrapper was written by
Rune Petersen for the efficiency gains this has over
QTime which does an expensive DST conversion internally.
I added back a timer restart in the elapsed() method
since QElapsedTimer doesn't always provide a monotonic
timer, and I implemented the addMSecs() method so this
could be used in all instances where the original
MythTimer was used.
Add unit testing for MythTimer. Refs #11255.