[usability] have shortcut for horizontal scrolling double as shortcut for next/prev (or back/forward)

[ghost]

On invoking h and l:

• if there is no horizontal scrolling possible, simply invoke next/prev (or back/forward)
• else invoke it once there is no further room for horizontal scroll in the specified direction -> This behavior is similar to what happens on the Mac trackpad with two finger horizontal swipes (which works for horizontal scrolling as well as prev and next)