De-duplicate planner default settings

All other default settings we get from subsurfacestartup.c's
initialization of the prefs struct. The planner had its defaults there
and in the retrieve of the settings from QSettings.

This changes so the defaults for planner settings will be read from the
defaults prefs struct as all other settings.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>