support targetGroupName for scale applications

[jonathanzong]

Similarly to Mark and Transform applications, allow user to set the target group for a Scale application (so that you can for example drag on one group to panzoom all the groups)

[jonathanzong]

1ed8084

tabling this because it's unclear what the desired behavior should be / whether it is something people would actually want in practical situations. also, would take some work refactoring the grid_ store/signals in demonstration.ts to work across views