legend.m: Handle builtin hggroup-based objects (bug #57260) * __contour__.m, __scatter__.m, __stem__.m, __quiver__.m, __errplot__.m: Set "__creator__" appdata. * legend.m: Use "icon" rather than "item" to name the icon objects (textitem_objects): Rename "texticon_objects". Use the new "create_icon" function to create objects. Use the icon object's appdata to store the creator for further triage. (create_icon): Decide what objects to create using the "__creator__" appdata of hggroups. Add listener to delete all legend objects for an item which has its peer objects deleted. (textitem_data): Rename "update_texticon_data". Call "update_icon_position" rather than returning an array of data (update_icon_position): New function that is aware of how to compute the position of the the icon object depending on the hggroup creator.

--- a/scripts/plot/draw/private/__quiver__.m	Tue Dec 03 17:50:59 2019 +0100
+++ b/scripts/plot/draw/private/__quiver__.m	Sun Dec 08 17:52:18 2019 +0100
@@ -182,7 +182,7 @@
     endif
 
     ## Must occur after __next_line_color__ in order to work correctly.
-    hg = hggroup ();
+    hg = hggroup ("__appdata__", struct ("__creator__", "__quiver__"));
     if (is3d)
       args = __add_datasource__ ("quiver3", hg,
                                  {"x", "y", "z", "u", "v", "w"}, args{:});