Enable undoing selection changes

[sidju]

Since it is very common to want to quickly print an area, probably tagged or the top of the file, and then returning to wherever you just were; it would be nice to be able to run =1 to set the previous selection.

• Command definition would be (.,.)=(0)
• The default of 0 would equate current behaviour, (set) and print current selection
• Any other number provided as argument would set and print the selection held that many steps back
• A step in selection is considered to be a change in selection, setting the same selection again isn't a change.

[sidju]

This is honestly a workaround for k not working on selections, not sure if this is worth it if that tagging is implemented but I'll leave it until that has been done.