Make account list scrollable

[baedert]

Some people have lots of accounts for reasons unknown to me.

BUT: there's no satisfying way of getting the popover sized correctly in gtk3. We obviously need to set propagate-natural-height of the GtkScrolledWindow to TRUE, but that means the popover will leak above the window border, which is especially bad on X11. The fact that the minimum size fits into the window is never considered. I'm going to look at this for gtk4 since I'm not going to change the behavior in gtk3.