drag-and-drop metadata: Support coloring merging

[joverlee521]

Allow drag-and-drop metadata to merge with existing metadata for fields that are used as coloring

auspice/src/actions/filesDropped/metadata.js

Lines 228 to 240 in 6370cc5

	/* restrict added colorings to those _not_ currently present on the tree. This could be relaxed. TODO. */
	for (const colorName of Object.keys(newColorings)) {
	if (controls.coloringsPresentOnTree.has(colorName)) {
	droppedColorings.add(colorName);
	delete newColorings[colorName];
	}
	}
	/* strip those droppedColorings out of `newNodeAttrs` */
	for (const colorName of droppedColorings) {
	for (const nodeAttr of Object.values(newNodeAttrs)) {
	delete nodeAttr[colorName]; // Note that this works even if `colorName` is not a property
	}
	}