User Chart and property "perspective" SEGV

.. lots of problems related to this, notably:

   * UserChart is no longer a GcWindow so doesn't have any
     properties registered.

   * Even if it was the property was not being registered
     by GcWindow or GcChartWindow anyway

   * The value was not being initialised so checking for
     NULL was kinda pointless (groan)

   * OverviewItems looked up the property and never found
     it, so crashes were avoided by accident.

.. One interesting point that was revealed during testing
   and debugging-- the UserChart program does not honor
   any filtering EXCEPT for the activity{ } function, which
   although it is not by design, is quite useful.

Fixes #4021

src/Charts/UserChartOverviewItem.cpp

@@ -22,7 +22,6 @@
 UserChartOverviewItem::UserChartOverviewItem(ChartSpace *parent, QString name, QString settings) : ChartSpaceItem(parent, name), space_(parent)
 {
 
-    // a META widget, "RPE" using the FOSTER modified 0-10 scale
     this->type = OverviewItemType::USERCHART;
 
     // default is a bit bigger