Some code, such as statics in ScGlobal, cannot(?) easily access ScDocument.
Moreover whether Calc is currently performing threaded calculations
is not really a state of a document but of Calc itself. Still keep
the state accessible via ScDocument for the pipe-dream future where it
possibly really will be a state of the document.

Change-Id: I2b112221e7fa1b2b4469cfd289fc201025585a5f
Reviewed-on: https://gerrit.libreoffice.org/54796Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>