Derive pqFontPropertyWidget from pqPropertyGroupWidget.

In Rosie, a custom ParaView app, we need to access the visibility of the
property group associated with this widget. Currently, most custom widgets
associated with a property group, derive from pqPropertyWidget, which does not
store a pointer to vtkSMPropertyGroup. This change, fixes this problem for
pqFontPropertyWidget.

Change-Id: I0594f606c3f975fcf090e13a2f0d11ab0f4736d1
Merge topic 'rms-plots' into next

0a09d3e6 Derive pqFontPropertyWidget from pqPropertyGroupWidget.