Have all formula cells start listening at once after the file load.

Rather than doing it individually.

Change-Id: I5ed55947b715bf6d7d61a1f8b751be7fdcf425fb
No point using the pimpl pattern here.

The whole class is already hidden in the source file.

Change-Id: Ib6157ae275217a95586735f74beee1700041a679
Use position hints for broadcasters when activating formula cells.

No reason not to do this since we are bulk-registering formula cells.

Change-Id: Ie0356c62a3c4698f5560272cb0c104f84cacde56