Add Corner Connector command

Added

• Add corner connector command
• Add script to download translations from CrowdIn using API
• Add German, Greek, French, Italian, Polish and Swedish translations

Changed

• Rename the icons following FreeCAD guidelines
• Match properties tooltips to wiki
• Update Spanish translations

Fixed

• Show commands' translated tooltips when using custom tooltip with icon
• Show height in the units preferred by the user
• Use correct parameter on ExtraBottomMaterial check

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Don't forget to take a look into doc files:

• README
• CHANGELOG
• CONTRIBUTING
• translations/README