Layer mask node - get merged mask

Thank's to the new layer system, the Layer mask node lost its initial purpose to be able to access layer masks. Currently it is somewhat a "convert a layer's base color to gray-scale node". I think there are two possibilities now:

1. Remove the node entirely
2. Change its behavior.

I vote for changing the behavior like this:

1. In case the selected layer is a mask layer it should behave just like it is the case at the moment. It simply takes the mask layer's values.
2. In case the selected layer is not a mask layer it should give the mask used for the layer. I.e. if there is only one mask there should be no difference to selecting the mask as a layer itself. If there are multiple ones the "merged" one (the one applied to the layer) should be used.
3. (If #797 is implemented) In case the selected layer is a layer group the behavior should be similar to 2.

By implementing it that way no existing materials should break and it would also unbreak the materials that broke due to the new layer system.

The issue names the Layer mask node and selected layer, mask-layer, and group cases, but no source file or test. Start by locating the node implementation and the existing layer-mask merge path; done means the three listed behaviors work without breaking existing materials, with group behavior addressed only if #797 exists.

Written by the indexing model from the issue text.