Compact and indent on JSON cell editor

New setting for enabling the following feature:
when JSON data is loaded in the JSON cell editor, the text is
indented. Before JSON data written back into the cell, the text
is compacted.

See issue #1173

Minor adjustment in preference label: removed ":" for consistency.

src/EditDialog.h

@@ -55,6 +55,7 @@ private:
     int dataType;
     bool textNullSet;
     bool isReadOnly;
+    bool mustIndentAndCompact;
 
     enum DataSources {
         TextBuffer,