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.

Note that the logic will be much easier than in middleware/reselect_moved_copied_strands.dart, because most of that logic is dealing with the fact that the strand address (more precisely the address of its 5' end) have changed, and we need complex logic to determine where the new strands are that "correspond" to the old ones.

In the case of this issue, the strands don't move. So we can just do the much simpler logic of populating the selected_strands field of [something in AppUIState, I forget where] with all the strands that have the same 5' address as the old strands whose field we updated.