- Reduce calls to millis()
- General cleanup of manage_heaters
- General cleanup of pid autotune
- Formatting here & there
- Macros to clean up and shrink ISR code (reduced by ~364 lines)

Added a second sensor and made them configurable.

Am I the only one who always need to change the thermistortable.h to add
a dummy table??
Ok, it is done. Just set the thermistor to 999 and it will always read
25C. Great for Testing or Development purposes.

- Fix bug in lcd_control_temperature_menu
- Remove test menu-item left in from multi-line command feature
- Replace literals with translatable strings
- Reduce lcd_preheat code redundancy
- Reduce menu macro redundancy
- Clean up formatting

Last commit before bed.

- Make this work per #1344

- Also apply cleanup for #1514

Slightly better layout for wide screens.

Introduced lcd_strlen() and lcd_strlen_P().
Replaced the old functions where necessary.
Reworked language_ru.h.

Speeded up test for zero length string in cardreader.cpp