Allow json arrays in settings for machine.*_sequence

[AmadeusW]

Allow json array for _sequence properties.
This allows for legible multi-gcode sequences, for example

  "machine": {
    "begin_sequence": [
      "G28 O",
      "G0 X70 Y30 F4500",
      "M291 P\"Ready to plot\" T0 Z1",
    ],

Array is supported only on input. Internally, the values are concatenated with newlines. That's apparent when the setting is exported:

    "begin_sequence": "G28 O\nG0 X70 Y30 F4500\nM291 P\"Ready to plot\" T0 Z1"