Add 'copy OS to USB' button to USB category
This adds support for copying the latest version of the OS to a given location. The primary use case is for copying the OS to a USB disk to be used to update the OS on another computer. This functions fairly similarly to copying an app to a USB stick. As with the ‘Copy to USB’ button for apps, only show it if it might work: if there's a collection ID configured on the OS remote which is a prerequisite for the copy to succeed.