Cell editor fixes and improvements

Valid JSON is detected as a new first-class data type. See issue #1173.

Text data length is always calculated in characters and not in bytes (UTF-8
gives different results).

Text in read-only mode is also selectable by keyboard and the caret is
visible. See issue #1123

src/EditDialog.h

@@ -67,7 +67,8 @@ private:
         Binary,
         Image,
         Null,
-        Text
+        Text,
+        JSON
     };
 
     enum EditModes {