[Suggestion] Add subchunk(SC) number to selection grid

[Dushess]

Main option is when you importing a schematic. Reason - if you managed to create large schematic file you have very funny journey of aligning it vertically while importing in MCEdit. Since you have 256 blocks only (16 subchunks) and poorly visible grid - you're basically unable to import first layer (1-16 SC), next layer (17-32) and others and keep your sanity at normal state.
I mean numbers at vertical selection box edge, right on bold lines.
Thanks.