feature/xpi0

If you don't need to install new or control old xpi plugins,
you can get rid of the "PlugInstall" interface by using xpi0
instead of the big xpi extension.

prev << "feature/xpi0"
next >> "action/ plugins"

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You cannot modify the README.plugins file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the README.plugins.Discussion.