fixes regression from commit 4467c67d,
where the new optimized code path failed to properly register
ScFormulaCell with ScExternalRefManager.
Introduces a new public method in ScExternalRefManager: insertRefCellAsTmpl
that registers a formula cell to listen to the same external references
as template cell
Introduces new property to ScFormulaCell: mbIsExtRef
that indicates that some external references are referenced by this cell

Also introduces new method ScExternalRefManager::hasCellExternalReference
(for use in unit test, will be posted immediately)

Change-Id: Iee07d7b51949fd31953b5fa662a213052eb85181
Reviewed-on: https://gerrit.libreoffice.org/16066Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>