MessageWindow: Add back tab scrolling

Daniel Brötzmannrequested to merge
tab-scroll into master

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)

Edited by Daniel Brötzmann

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