shell-perf-helper: Add an option for continual redraws

Add an option for windows created with Scripting.createTestWindow()
to continually redraw themselves; this is for testing performance
of application updates.

https://bugzilla.gnome.org/show_bug.cgi?id=732350
ShellPerfLog: Use monotonic timestamps for logging

Use g_get_monotonic_time() (that is, CLOCK_MONOTONIC) timestamps,
rather than gettimeofday() values.

https://bugzilla.gnome.org/show_bug.cgi?id=732350