Starting the SwDocIdle in StartIdeling() just reintroduces the
bug fixed in commit 401cba4c ("tdf#116370 cleanup Writer idle
job handing"), so trading one bug for the other.

For the real solution we have to handle two states:
1. The SwDocIdle being active
2. The SwDocIdle being blocked

For the first state we can just use the active state of the Idle
itself. Since it's not a AutoIdle, it will be turned of, if
invoked. Either some Idle handler will want to be run it again or
some other thread might start it again, if needed.
Since we're now tracking the wanted Idle state via the active task
flag, we can drop the explicit mbStartIdleTimer handling.

As a result of the first change, we can't stop the Idle anymore
when blocking it. But the Idle itself checks its ready state via
IsDocIdle() in its UpdateMinPeriod function, so we have to add
the blocking state to IsDocIdle().

Change-Id: I33017a8e07208a6a3bce761cc899ac3ca944cd61
Reviewed-on: https://gerrit.libreoffice.org/59586
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>