improve textview width calculation

small window sizes got unintentional line wrapping.
remove the ceil on the cellSize width and remove the margin around the
text view
disable line wrapping on NVMTextView
fix autolayout constraints in NVMWindowView.xib
reduce NSLayoutConstraint priority in NVMSplitView

This resolves auto layout errors if the subviews don’t have the same
width/height.
move cellSize from to NVMClientWindowController
implement basic separator views