Fix histogram visibility

[vallsv]

This PR fixes the setSideHistogramDisplayed(False) when the widget or one of the ancestor is not visible.

isVisible() is one of the property which is not symmetric to setVisible().

As result isSideHistogramDisplayed was false is some cases for false reason.
Which was inhibiting setSideHistogramDisplayed.

Changelog:

• Fix ImageView histogram visibility

[vallsv]

The following code from ImageStack`looks to have the same problem

    def urlSelectionTableIsVisible(self):
        return self._urlsTable.isVisible()