Support custom fields for "Quick Labels"

[ceremcem]

Currently Quick Labels are in this format:

    bom:
        foo:
            '500ohm':
                "x":           
                    1: "aaa"
                    2: "bbb"

Labels are supposed to be strings. However they should also support objects, like for "Cable definition" purposes:

    bom:
        Dsub_9_vertical:
            '':
                "rs232":           
                    2: 'tx': connect-to: "Omron.RX(3)"
                    3: 'rx': connect-to: "Omron.TX(2)"   
                    5: 'gnd': connect-to: "Omron.Gnd(9)"
                    # no connection
                    1: 'nc'
                    4: 'nc'
                    6: 'nc'
                    7: 'nc'
                    8: 'nc'
                    9: 'nc'