Issue #9156: Initialize mask from grascaled copy of chosen layer.

I didn't fully understand some of the code, so I left NULL in some functions as source_layer argument.

and my questions:

1. replace *channel and *source_layer in gimp_layer_create_mask with *void argument and then cast it to required type based on add_mask_type.
2. rename combo in layer_add_mask_dialog_new to combo_channel to follow naming.

I think these are still mistakes that I need to fix. That's why I leave some my notes in merge request. Is it normal, or I need to remove them all the time, even if 100% there will be further edits?