gtkscrolledwindow: Change lifetime of kinetic scroll helpers

gtkscrolledwindow: Change lifetime of kinetic scroll helpers

In order to properly accumulate scroll velocities, we need to keep
the kinetic scroll helpers after we have possibly stopped them
in the process of initiating a further scroll flick.

So, instead of stopping (and destroying) those helpers on scroll-begin,
keep them until the next scroll-end if a scroll was initiated before
kinetic scroll finished. This way we can fetch the last velocity when
calculating the extra kick.

In order to ensure the helpers don't live beyond what it is expected,
we now also remove them after a finished hold event.

Fixes the accumulation of scrolling velocity on consecutive scroll
sequences.