Read/write and loading all works

Finally got the item model working and it now loads and saves plist
files, and the resulting file is readable by XCode.

Have also begun implementing custom delegates for YES vs NO on the
Boolean entries, and for the Kind, eg String, Array and so on.

Still to do - change the PlistElement based changing the Kind in column
1, eg when choosing a String item and changing to an Array, modify the
PlistElement in the data structure to suit.

Also hook up the standard text editing for String objects to the large
text area.
Implemented adding and removal of items

Also tried with delegates to get it looking and working better but
still very clunky with key navigation.  Also cannot get the large
editor pane working well.

Added buttons there to update and reset - might be better approach.