Compute panel zone sizes.

def _compute_zones_sizes(self):
        """Compute panel zone sizes."""
        # Left panels
        left = 0
        for panel in self.panels_for_zone(Panel.Position.LEFT):
            if not panel.isVisible():
                continue
            size_hint = panel.sizeHint()
            left += size_hint.width()
        # Right panels
        right = 0
        for panel in self.panels_for_zone(Panel.Position.RIGHT):
            if not panel.isVisible():
                continue
            size_hint = panel.sizeHint()
            right += size_hint.width()
        # Top panels
        top = 0
        for panel in self.panels_for_zone(Panel.Position.TOP):
            if not panel.isVisible():
                continue
            size_hint = panel.sizeHint()
            top += size_hint.height()
        # Bottom panels
        bottom = 0
        for panel in self.panels_for_zone(Panel.Position.BOTTOM):
            if not panel.isVisible():
                continue
            size_hint = panel.sizeHint()
            bottom += size_hint.height()
        self._top, self._left, self._right, self._bottom = (
            top, left, right, bottom)
        return bottom, left, right, top