allow user to specify which structure or sequence is used as reference when making structural superposition

There are a number of places where the user can implicitly specify the reference structure for superposition - either by mousing over a specific sequence prior to using the 'View all . structures' option to open a superposition or by adding a new structure to a structure already displayed (which is then used as the reference).

The knowledge of which is the reference structure, is - however - not displayed anywhere. A menu option allowing the reference structure to be picked when superposing would clarify this.