Fix autocompletion for items in folders

This provides useful autocompletion when typing `Foldername/jobname` into an *Add Item* page's *Copy from* text field. 

The problem in the current implementation is the line `if (value.startsWith(group.getFullName()))` which prevents descending into the folder 'F' once the user typed 'F/'. Deprecated, because someone might use it programmatically.

There's a working implementation already, so reuse it, but allow specifying an `ItemGroup` context. Unused in Folders plugin AFAICT, but might be useful anyway.