Planner: wild guess attempt to fix a crash on Mac

Staring at the stack trace it seems that it gets into an infinite
recursion when trying to recalculate after being alerted to a change on
the ruler. I cannot recreate this here (not on Linux, not on Mac), but
here's a random attempt to prevent the issue: simply refuse to recalculate
the ruler while in Add or Plan mode.

Crude, but might show us if this really is the issue. Otherwise it's easy
enough to revert this change. The qDebug() in there should tell us if
people on a Mac do indeed see this even without moving the ruler around in
Add or Plan mode.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>