app: allow controlling performance-log parameters through the UI
When recording a performance log, allow setting the log parametrs through the file dialog. Currently, this includes the sample frequency, and the option to include backtraces. These options are still controllable through the GIMP_PERFORMANCE_LOG_SAMPLE_FREQUENCY and GIMP_PERFORMANCE_LOG_BACKTRACE environment variables. When set, the variables override the values entered through the UI.
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