Issue #8349: Add alpha channel to layers for transparency masks

This attempts to implement a solution for #8349.

Per the report, you get different behavior when adding an alpha channel layer mask or transfer alpha channel layer mask, based on whether the layer has or doesn't have an alpha channel.

One of the two suggested solutions was to add an alpha channel to a layer when those mask types were selected. As GIMP already does this when applying a mask (https://gitlab.gnome.org/GNOME/gimp/-/blob/master/app/core/gimplayer.c#L2248), it seems to make sense (and is less disruptive to altering multiple layers than throwing an error message).

@Ofnuts, hi! Has your thoughts on this changed any since the initial report, or do you still think this is a good solution?