Only call the root or selected container selection, not both.
Merge pull request #539 from jaycee/units-on-root-container-have-issues

Fixes errors on dropping units on root container.

Errors occurred b/c in cases where the root container was the drop target, the code attempted to select a container *after* `selectMachineToken` had already selected the root container.

This updates `selectMachineToken` to call `selectRootContainer` or `selectContainerToken` depending on an optional containerToken parameter.