Initial support for multiple Nests

Multiple Nests are supported by letting the user see the list of
available Nests and choose an active one. The active Nest will be stored
in the workflow data area. The active structure can be chosen in the
same way.

Internally, functionality has been split between Account, Structure, and
Nest classes based on how Nest appears to divide up the work.
Remove struct command, bugfixes

- Removed struct command since it was redundant
- Fixed a bug that prevented the workflow from sending commands to Nest
- The temp command will now show the target temp as you type it