I ran a profiler to determine where the runaway CPU usage appeared, and it
showed up as dominated by calls to msleep() in SenderThread.  I did the
standard replacement of the busy loop with a mutex and wait condition, which
solved the problem.  Initial interactive testing showed no degradation of
timing accuracy in playback.

This means that the program doesn't need to look for the file in a particular
local path since it's embedded in the executable (it was already there anyway).
QMediaPlayer doesn't reliably support loading from resources but QSoundEffect
does, and the Qt docs recommend using the latter for this kind of purpose
anyway.

…plicitly; also tightened timing of the metronome by removing silence from the start of the click file

… >= 5.13