add struct for a viewWindow, encapsulates just one of the windows of a view. Add callbacks + new init call

For expediency, commented out some function calls where we'll likely be doing things differently when we actually come to draw features

For expediency, commented out some function calls where we'll likely be doing things differently when we actually come to draw features

Moved pane-splitting code from zmapWindow. Moved public functions accessing private ZMapPane members to here