some tweaks for the GUI / handling

[coldtobi]

Hi Luca,

Kudos for this project! The Maiskolben has become my favourite soldering tool ;-)

I've hacked a bit on the software, but I fear not everything of it might be interseting for you. So instead of making PR, I think it is better to give you a pointer to the changes and if you think something would be useful for to merge, please let me know and I will create feature based PR.

You'll find the changes on here: https://github.com/coldtobi/Maiskolben/tree/tobi

Changes you might be interested in:
- Standby button handling:
2 changes:

• if station is off, ignore to go to standby mode (it would anyway not heat if not turning it on)
• I prefer that a short press of SW_STBY, when the station is off, turns it on.
  This first one is commit cfbb807, the second contains both and is commit b16b7bc

More changes:

• Some eye-candy for the color defintion.
  Just a small helper macro to be able to define the colors out of rgb components, without the need
  to know how many bits are for which color... commit 83a6a96

• A Fastboot feature (compile time switch)
  Bikeshedding again, I'd like not to wait in the boot.. So this will be just a bit quicker, in the cost of that the boot screen is only shown for a short time. Commit afa4ae6

• I've got an external BMS / Charging circuit using LiFePO4 cells. For this I hacked together some coarse support to show a battery symbol with the current charge state. As my hardware is a bit wicked (I had the double-FET dying on my board, killing also a trace, so I cannot tell if that modificatin will play nice on "stock" hardware). (Those are several commits, sorry cannot name them here, but looking at the diffs should give enough hints, I hope.)

• The default voltage measurement cycle of 200ms is quite slow, especially with the FIR filter... So it takes quite some time to detect e.g fallback from external power to USB power. Commit 54647ea makes that faster.

Let me know if there are any questions!