Tube runtime counter for vacuum tube display builds

[vladkorotnev]

Basically what it says.

AC:

• I can enable the feature with HAS_VACUUM_DISPLAY flag in device config
• I can see a tube runtime counter in the Settings → Status menu
• The runtime counter is in the format of Hour:Minute:Second
• The counter only counts as long as the tube has the heater enabled, no matter whether the pixels are lit or not (set_power(true) was called, no matter what the value of set_show is or the rendered picture is)
• The counter persists across firmware upgrades and reboots