Extensions - Create/adapt Addons to Extensions chapter

[Draise14]

New extensions added. A chapter should be adapted from the addons chapter, no-longer a thing.

Features:

• What is it
• Opting in
• Difference between legacy addons and extensions addons
• Online databases
• Third-party repositories
• Installing local extensions
• Installing online extensions
• Installing legacy addons
• Installing online themes
• Uninstalling addons

Details

SHA-1: 63599db59862aace9a1edfc102471776477bb280 * UI: remove "All" option which showed add-ons and themes by default Showing all kinds of extensions at once isn't all that useful, only show one type at a time. 

Details

SHA-1: c9a99c7eeb4293f3fee21bd10865c6e36973b05b * Extensions: revised welcome message + behavior Make it possible allow online access directly from the welcome screen. Ref !122511

[Draise14]

Thanks @ReinerBforartists looks like you did most of this one.