Added to grbl_post optional bCNC block header for every job operation

[spanner888]

Dev forum discussion https://forum.freecadweb.org/viewtopic.php?f=15&t=42956&p=366887#p366887
Added in minimal way, ie did not convert ALL existing comment blocks to
bCNC format, as either existing seemed preferred, or already wored as
bCNC header blocks.

Enabling with --bcnc argument (not default) adds () gcode comment blocks
in bCNC format to provide colapsable blocks that can be dragged in bCNC
to edit or alows en/diable in the job.

Thank you for creating a pull request to contribute to FreeCAD! To ease integration, please confirm the following:

• Branch rebased on latest master git pull --rebase upstream master
• [Sorry - Do not have suitable build computer (Stuck in ChinaATM) Also change to to single Path post processor file and grbl output has been tested in bCNC] Unit tests confirmed to pass by running ./bin/FreeCAD --run-test 0
• Commit message is well-written
• [na] Commit message includes issue #<id> or fixes #<id> where <id> is the associated MantisBT issue id if one exists

And please remember to update the Wiki with the features added or changed once this PR is merged.
Note: If you don't have wiki access, then please mention your contribution on the 0.19 Changelog Forum Thread.

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