[GNC] Scrolling of the accounts tabs

[AC]

So in 2.6 land I used to be able to scroll the list of account tabs with 
the mouse wheel when the list (vertical list, to the left of the 
register) was taller than the available space without causing the 
currently selected account to change.  The arrows at the top and bottom 
of the list of tabs would change the selected tab and work its way 
through the list.

In 4.8 land the arrows still function as before but scrolling the tabs 
with the wheel is gone. I'm only left with the context menu to browse 
through the list. I'm going to assume yet another GTK-ism due to the 
switch to the later GTK because I don't see a setting for that.