Adding 'copy' action to the item browser.

[fukuchi]

This patch allows the user to duplicate an item from the admin dashboard. While the same feature is already implemented as a plugin (ItemCopy), this implementation is much simpler. Plus, the copy action can be used to implement "default values" (see this discussion).

Here's a screenshot.

This patch uses 'fa-copy' icon introduced by #1501. If #1501 will not be applied to the upstream, please apply the patch included in cc300f468b2 to iconfont.scss.