MessageWindow: Add back tab scrolling
This adds back tab scrolling in the MessageWindow's notebook. (Note: mouse pointer has to point on the tab bar, not on a tab itself). This feature was intentionally removed in GTK, see https://bugzilla.gnome.org/show_bug.cgi?id=630226.
I think this is a useful addition, especially to improve tab scrolling for people using a left/right-positioned tab bar to fit more tabs.
Fixes #9172 (closed)