HaikuDepot: Fix Scrollbar

The scrollbar on some areas is misaligned
with the window frame. This change will fix
this. The class that implements the scroll-
view is also shaded by two class definitions.
This is also fixed by breaking the definition
into its own file.

Fixes #16169

Change-Id: I71372906e165672c9b697ab98181ecfb1b09fb43
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4849
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>