ui: fixed exception colorizing cells

On some distros (Fedora 37), on the very 1st launch of the GUI, it
crashed with the error:
 "object of type QCommonStyle has been deleted"

The only way I've found of getting rid of this error is by obtaining this
object on every paint() call.

ui/opensnitch/customwidgets/colorizeddelegate.py

@@ -48,6 +48,7 @@ def paint(self, painter, option, index):
         defaultPen = painter.pen()
         defaultBrush = painter.brush()
 
+        self._style = QApplication.style()
         # get default margins in order to respect them.
         # option.widget is the QTableView
         hmargin = self._style.pixelMetric(