implement NVMWindowViewController
warp NVMTextView inside a NSView instead of NSScrollview

* the NSView clips the text view
* we don’t need scrolling (now) as only the visible lines are inserted
into the textview
* removing the scrollview speeds up resizing (unintentionally but a
good side effect ;)
implement basic split views

When mixing horizontal and vertical splits there are some auto layout
constraint errors. Vertical splits don’t add up to full window with,
because the 1 char separators are missing.