WindowServer+LibGUI: Pass the set of mime types being dragged to client

Previously the client would only learn the mime type of what was being
dropped on it once the drop occurred. To enable more sophisticated
filtering of drag & drop, we now pass along the list of mime types being
dragged to the client with each MouseMove event.

(Note that MouseMove is translated to the various Drag* events in LibGUI
on the client side.)

Libraries/LibGUI/Model.cpp

@@ -71,7 +71,7 @@ ModelIndex Model::index(int row, int column, const ModelIndex&) const
     return create_index(row, column);
 }
 
-bool Model::accepts_drag(const ModelIndex&, const StringView&)
+bool Model::accepts_drag(const ModelIndex&, const Vector<String>&)
 {
     return false;
 }