Couldn't leave to finish this; four-split 3d view now
syncs the ortho views to a box-view. Brings back good
old Amiga Sculpt4D feeling! :) What's not done is to
use the box to clip away invisible stuff.
Note that it still allows to switch top-bottom,
left-right or front-back. The 4th window is entirely
free.
Test: use shift+alt+ctrl+S. In due time will all go
the headermenu, configurable, of course.
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