Fix/heat cool bugfixes

[lossanarch]

Thanks for the card @fineemb!

Since I use heat/cool mode a lot, I noticed that there were a few issues:

1. Added dual_state bool to track being in a state that makes sense to support dual temp adjustment (this.dual wasn't enough on it's own as it applies even when the hvac mode is a single-temp mode like heat or cool.)
2. Fixed the logic in heat_cool and off modes to put the temp labels back in place
3. Reminded it to load the mode icons
4. Set from and to values correctly when in heat_cool mode

Just now I've realised my prettier config in vscode got a hold of it, and that's the rest of the changes.

I've had this running a few months now in use with heat pumps that run in heat_cool most of the time and electric heaters that run in heat mode, and everything seems good enough, so I don't think I've introduced any horrible bugs.

If there's anything you'd like changed let me know.