In the Asset Browser while showing the //Current File// asset library, we now show a menu to choose between {nav Duplicate} or {nav Reuse}. This is similar to the menu for external asset libraries, that allows selecting between {nav Link}, {nav Append} and {nav Append (Reuse Data)}.
{Append (Reuse Data)} and {nav Reuse} should behave as consistent as possible with each other.
Unfortunately we can't currently do a generic duplication in the more asset-specific code. Dupliation is ID type specific and in some cases there also needs to be special handling (e.g. local objects are always duplicated, because otherwise a drop would only transform the object). So the duplication has to happen in the operators executed on dropping.
Basic idea of the patch:
* Only the Asset Browser knows about the option, so it has to let the dragged item know if it wants it to be duplicated or reused on drop.
* On drop, the `copy()` callback of a drop-box sets a `"duplicate"` property of the operator, if available, based on what the drag item requests.