Bug 908162 - Removes Button module's dependency from Sidebar module

- removed `sdk/ui/button` dependencies and `sdk/ui/state` dependencies
- removed `icon` property
- removed `CustomizableUI` dependencies
- thrown a new Error if the ID is already used in `create` method of `ui/sidebar/view`
- updated tests
- replaced metadata: sidebar is not compatible only with australis now
- replaced deprecated `for each` loop with `for..of`
- disabled `testSidebarIsOpenInNewPrivateWindow` in `private-browsing-supported`