assigning properties to several selected strands should leave strands selected

[dave-doty]

Select several strands and assign some "property" to them all of them, for instance, vendor fields:

Afterwards they should remain selected, but they are unselected:

This afflicts other "assignments" that can be done to multiple strands, e.g., assigning names.

[dave-doty]

Look at the file middleware/reselect_moved_copied_strands.dart for an example of how to do this, and read the comments for why it is necessary.