exp_adminui 1.0.0.50: the admin tabs name their icons, modern README

• Updated: The Media library, User accounts and Webshop tabs name their icons in the adminui siteaccess's
  menu.ini (Icon=images, Icon=people, Icon=box-seam), with the comment the kernel's menu.ini uses for the
  setting. They are the icons an admin4 header draws for these tabs; the adminui side bar keeps its own Font
  Awesome icons.
• Updated: A modern README: who the extension is for, requirements, installation with Composer and by hand,
  every setting with an example block, usage, permissions, upgrading, known limits, troubleshooting, tests and
  contributing.
• Updated: composer.json credits Netgen, links the documentation and names the extension directory for both
  installers; extension.xml lists the bundled third-party software and the dependencies on ezjscore and expui.
• Added: This changelog.