Changed snap toolbar behaviour:
- create a list of available snaps (Gui.Snapper.snaps)
- make it consistent with Snap Gui Commands (in gui_snaps module)
- create a list of active snaps (Gui.Snapper.active_snaps)
- refactor the isEnabled() method to allow it to check if the given snap is in Gui.Snapper.active_snaps and not if the snap toolbar button isChecked()
- updated and reordered the new list of gui snap commands in draftutils.init_tools and used it as a base to refactor the creation of draft toolbar
- updated all the draft snap gui tools to make them control the toolbar buttons directly
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[Draft] Attempt to make base classes compatible with super on py2


[Draft] Refactored imports of snapper and snaps gui tools


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Cleanup after @vocx-fc review

thanks to vocx revisions