Allow group selection opacity changes in g.gui [Feat]

[ericrpatton]

Is your feature request related to a problem? Please describe.

It would really be nice to be able to control the opacity of a group selection of layers in g.gui all at once, much the same as the ability to check, uncheck, and remove a group selection.

Describe the solution you'd like

Select a series of layers in the Layer panel of g.gui, right-click and select 'Change opacity level', which would bring up the same dialog box that you get when a single layer's opacity is changed. Except in this case, all of the selected layers would adopt the same opacity level that is chosen on the slider in the dialog box.

Describe alternatives you've considered

The only alternative is changing the opacity of each layer individually, which is extremely tedious is you have anything more than a few layers loaded in g.gui.

[ericrpatton]

I was going to add the 'g.gui' label to this issue, but I can't find any way to do that.