Show field descriptions on FilterAdder.js

[dshishkov]

• NestedPicker: add support for showing option description section/panel if at least one option has field description set
• ModalPicker: add modalClassName prop to allow targeting for styling
• FilterAdder: pass modalClassName prop as filter-adder by default to allow targeting for styling the modal within the FilterAdder

Used in Spark with some greyvest theme added style overrides

.filter-adder .default-modal-wrap {
  border: 0;
  max-width: 1000px;
}
.gv-picker-description-container {
  background: ${bg};
}
.gv-picker-description-container > em {
  color: ${muted};
}
.gv-picker-description-container > div {
  color: ${active};
}

[decrapifier]

	Warnings
:warning:	The README has not been updated. Please update the README.

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