logging, format strings, tests don't change settings, split tests, --gst- arguments

[sezanzeb]

• uses the logging module now, since that is what many python modules use - makes the code possibly more familiar - and it allows to log to files and such if that is required at some point.
• correctly forwards --gst- arguments. For example soundconverter --gst-debug-level=3. See https://gstreamer.freedesktop.org/documentation/application-development/appendix/checklist-element.html?gi-language=python#debugging and gst-launch-1.0 --help-gst
• does not modify the users setting during make test using a memory backend (unittests #30 (comment)). The way how the code gets the Gio.Settings object had to be changed a bit for this.
• moved from '%s' % 'string' to the newer '{}'.format('string') formatting almost everywhere
• clarified differences between argv and gio settings
• put audio-format related code from settings into separate file
• splitted test file into multiple files
• fixed changing the number of jobs required a restart
• fixed deprecation warnings for Gtk.Dialog
• rearranged readme a bit
• limit-jobs is treated as boolean now instead of int

example showing gstreamers debug messages and our new colored debug logs. The lines saying "WARN" in yellow are gstreamers debug messages.

gstreamer really logs a lot of things with increasing verbosity (up to 9). I removed the debug logs for the message types at some point because they were actually a bit too much, so the screenshot is a bit outdated.

[sezanzeb]

the --gst-debug-level=3 argument may possibly help debugging https://bugs.launchpad.net/soundconverter/+bug/1881376

[kassoulet]

Amazing work, thank you !