Fix in extdatetime.h: overloaded ExtDateTime::fromString() functions

Fix in extdatetime.h: overloaded ExtDateTime::fromString() functions
were ambiguous; fixed by removing a default parameter from one of them.

KPlotWidget must be derived from QFrame rather than QWidget, otherwise
it is not usable in Qt designer.  In designer, you place a custom widget
by using the widget's baseclass, and then "promoting" it.  However,
designer does not provide a QWidget!  So, it would seem that custom
widgets cannot be derived directly from QWidget.  This may be a bug in
designer.

CCMAIL: kde-edu@kde.org
CCMAIL: kstars-devel@kde.org

svn path=/trunk/KDE/kdeedu/libkdeedu/; revision=489303