Pick up the GPX 2.6.8

Attempt another fix for hang on cancel

Pick up a new GPX to solve filename non-ASCII bug

Extra precaution to prevent handing any non-ascii character to OctoPrint

Rename gpx python module to avoid collisions

in preparation to allow wheel installation of the binary extension
to possibly allow install without python-dev in the future

Eat 'None' lines out of .ini files

Pick up latest GPX

Treat 'None' the same as 'undefined' in json payload

Switch start print detection strategy to

use gcode script insertion rather than relying on line number behavior

Fix AttributeError when current job is None