Add 'Tap tempo' button to 'Edit tempo' dialogue

Add a 'Tap tempo' button to the 'Edit tempo' dialogue box that uses
gettimeofday() to time the interval between successive clicks and sets the
beats-per-minute appropriately.
Make 'Tap tempo' set the bpm to a running average

Keep a running average of the interval between clicks on the 'Tap tempo'
button, and use that average to set the bpm value.
Make tap tempo use g_get_monotonic_time(), and fix minimum BPM

gettimeofday() is not guaranteed to be monotonic: use
g_get_monotonic_time() instead.

Also, fix calculation of slowest tap tempo BPM so that the slowest tempo
which can be set by tapping is 10 BPM rather than 240.
Report an error when post-export hook fails.
Make $PATH search in SystemExec actually work

Make searching of $PATH work when a SystemExec  is created from a
command-line, by making sure that argv[0] is set to the discovered path.
Left-align label for post-export hook script entry.