As was the original idea, for some reason abandoned, and as ScAreaLink does.

The main advantage is that ImportData() is not called anymore with
invalid mpScDocument (when we close it).
There is surely something going on with SolarMutex and it just works better.
We do not need to use our own SolarMutexGuard.

Change-Id: I09256a18ffa14606fc8e06b968016bbc0562a6fa