+ * Validates user selection and enables the 'Add' and 'New View' buttons if
+ * all parameters are correct (the Add button will only be visible if there is
+ * at least one existing structure viewer open). This basically means at least
+ * one structure selected and no error messages.
+ * <p>
+ * The 'Superpose Structures' option is enabled if either more than one
+ * structure is selected, or the 'Add' to existing view option is enabled, and
+ * disabled if the only option is to open a new view of a single structure.