Add an option to record a performance log through the dashboard.
The log contains a series of samples of the dashboard variables, as
well as the full program backtrace, when available.  As such, it
essentially acts as a built-in profiler, which allows us to
correlate program execution with the information available through
the dashboard.  It is meant to be used for creating logs to
accompany perofrmance-related bug reports, as well as for profiling
GIMP during development.

The sample frequency defaults to 10 samples per second, but can be
overridden using the GIMP_PERFORMANCE_LOG_SAMPLE_FREQUENCY
environment variable.  Backtraces are included by default when
available, but can be suppressed using the
GIMP_PERFORMANCE_LOG_NO_BACKTRACE environment variable.

Logs are created through the new "record" button at the bottom of
the dashboard dialog.  When pressed, a file dialog is opened to
select the log file, and, once confirmed, data is being ...