[P073] Add support for 74HC595 displays

[tonhuisman]

From a forum request

Features:

• Support 74HC595 displays with 2, 3, 4, 5, 6, 7 and 8 digits, 2+2, 2+3/3+2, 3+3, 3+4/4+3 and 4+4 (sequential) combinations.
  • Support for 74HC595 multiplexed displays (using just 2 74HC595 chips) is disabled for now, as the required continuous update-speed is too low. ) Working on a different solution. Seems to work somewhat acceptable. (4/6/8 digit displays).
• For 2 and 3 digit displays, the 'Hide ° for Temperatures' setting is activated by default (no override).
• Updated documentation
• This new feature is disabled for ESP8266 builds for now, because of size reasons.
• Add option for blinking dot instead of colon for blinking time

TODO:

• Testing:
  • 2 digit displays
  • 3 digit displays
  • 4 digit sequential displays
  • 4 digit multiplexed displays (tested but not working good enough yet) Acceptable.
  • 6 digit multiplexed displays (no displays available, tested on 8 digit display).
  • 8 digit multiplexed displays (tested but not working good enough yet) Acceptable.
• Update documentation for blinking dot

For better results with 74HC595 matrix displays (higher refresh-rates) probably extra hardware is needed, like a dedicated Arduino Nano, with the only task of displaying 7segment-display content.