Created by: vicdoval
This PR brings support for external addons inside sverchok. All the functionalities it brings are applied in new Extra Addon Sverchok-Open3d (Point Cloud and Triangles Mesh tools)
Option added:
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Include Custom menu to the Sverchok Add menu (with subcategories and separators)
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Link to documentation from regular UI buttons
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Examples folder added to the Examples menu
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Download the addon from from Sverchok Preferences
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Check for Updates and Update the addon from UI
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Code changes complete.  - 
Ready for merge.