PM in RA and Dec are essentially derivatives of the RA and Dec taken

in the J2000.0 reference frame (hopefully). Even otherwise, the
difference should be small (and "second order"). One must however be
aware that this kind of a difference does accumulate over time, but it
should still be okay.

However, sometimes, this method is called before dec() is actually
computed! So, replacing dec() by dec0().

Should backport to 4.10, master.

CCMAIL: kstars-devel@kde.org
(cherry picked from commit 98eac0c5)