AbstractPropertyWidget's onPropertyChanged slot is invoked from non-GUI
threads. Previously, it would call updateWidgetFromProperty directly,
which resulted in Qt widgets being accessed from non-GUI threads. This
in turn led to random crashes when properties were modified quickly and
repeatedly from the GUI.

Now we invoke updateWidgetFromProperty via a signal-slot connection with
an internal signal, s_propertyChanged. This makes Qt take care of
queueing slot accesses in the GUI thread for us.

Fixes #36