Change NSArray properties to copy ownership.

Also, changed setSectionTitles: to copy the new sectionTitles into the _sectionTitles ivar.

Note: Considered removing the `if (_sectionTitles != sectionTitles)` wrap from setSectionTitles:, as it's no longer needed (unless someone does a `self.sectionTitles = _sectionTitles`, which is unlikely).  However, this could to unwanted side-effects since setNeedsDisplay/setupAccessibility would still get run even though no values have changed.  As such, it's probably safer to just leave it in.