[autosave] selecting a state triggers an autosave

[CSchank]

Selecting a state currently triggers an autosave. This isn't bad from a data integrity point of view, but it is technically inefficient and could confuse the user since they might wonder what they changed.

[CSchank]

This is because it thinks we're dragging the state. We should store the starting position in the drag program state and only save if something changed. (src/Building.elm:125-130)