Fixed the ignoring of manually named "Main" page so we collapse it too.

The "Main" page was ignored from group collapsing since it was renamed manually for pages with no name(name=nullptr). So whenever we find an empty name we rename it to Main and add on the set with other pages. Also we fixed the issue when getting the first page from the group, as we were storing them in an unordered set and it does not guarantee that the first page we get from it is the first one inserted.