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`
Merge pull request #1216 from ZER0/sidebar/908162

fix Bug 908162 - Removes Button module's dependency from Sidebar module r=@erikvold