ungroupColumns method
Removes one outline level from columns fromColumn-toColumn and
un-hides them.
Implementation
void ungroupColumns(int fromColumn, int toColumn) {
if (fromColumn < 0 || toColumn < fromColumn) return;
for (var c = fromColumn; c <= toColumn; c++) {
final cur = _columnOutlineLevel[c] ?? 0;
if (cur <= 1) {
_columnOutlineLevel.remove(c);
} else {
_columnOutlineLevel[c] = cur - 1;
}
_columnHidden.remove(c);
}
_columnCollapsed.remove(toColumn + 1);
}