Call updateHeadsUpDisplay in fromVariantMap

[alxvth]

ScalarAction::sourceDataChanged is not emitted when loading a scatterplot from a project. Therefore

    connect(&_settingsAction.getPlotAction().getPointPlotAction().getSizeAction(), &ScalarAction::sourceDataChanged, this, &ScatterplotPlugin::updateHeadsUpDisplay);
    connect(&_settingsAction.getPlotAction().getPointPlotAction().getOpacityAction(), &ScalarAction::sourceDataChanged, this, &ScatterplotPlugin::updateHeadsUpDisplay);

from #193 don't trigger updateHeadsUpDisplay.

We have to manually invoke updateHeadsUpDisplay in fromVariantMap