When units are selected and a building is placed, only place building and do *not* send units there.

From #287 

```
there should be a separate cursor state to place buildings. now, if I have a unit selected and I place a building, the unit will head to the building - the original game kept building placement and unit movement separate
```

## TODO
- [x] fix #367  (found while playtesting, caused by trying to fix this issue)
