set layer scale visibility for several layers

This allows setting the scale visibility of several layers at once.
This is done similarly to "set CRS of layers": it adds a layer menu entry and a context menu entry.

If no objection is raised by feature freeze, I will merge this.

@wonder-sk: will this be a problem for the legend refactoring merge? 
I implemented ```void QgsLegend::setScaleVisibilityForSelectedLayers( bool hasScaleVisibility, double minScale, double maxScale )``` similarly to ```QgsLegend::setCRSForSelectedLayers```. If you have a working branch, I can rebase on top of it to avoid any extra work.