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 recorded to
the selected file.  Recording is stopped by pressing the "record"
button again (we use a highlight to indicate that recording is
active.)

While recording, the "reset" button is replaced with an "add marker"
button, which can be used to add event markers to the log.  These
can be used to mark events of interest, such as "started painting"
and "stopped painting", which then appear in the log as part of the
sample stream.  Markers are numbered sequentually, and the number
of the next (to-be-added) marker appears on the button.  Shift-
clicking the button adds an empty (description-less) marker, which
is only identified by its number; this can be used when markers
need to be added quickly.

The log is an XML file, containing some extra information (such as
the output of "$ gimp -v", and symbol information) in addition to
the samples.  The data in the file is delta-encoded to reduce the
file size, meaning that samples (as well as some other elements)
only specify the changes since the previous sample.  This adds a
necessary decoding step before data can be processed; the next
commit adds a tool that does that.

There are currently no tools to actually analyze the data -- that's
still TBD -- but at least we can start gathering it.