uisupport: Forcefully disable horizontal scrollbar in BufferView

uisupport: Forcefully disable horizontal scrollbar in BufferView

Qt 5.10.1 severly broke QHeaderView and by extension, QTreeView,
resulting in a horizontal scrollbar being shown. In order to avoid
making this issue too obvious for people using that version of Qt
(including the builds currently generated by our CI), force the
scrollbar to be hidden in BufferView.

The contents can still be scrolled sideways e.g. with horizontal
mouse wheel emulation, but that should not hit many users in
practice.

Earlier or later versions of Qt seem to not suffer from that issue,
but always hiding the scrollbar doesn't hurt either way.

(cherry picked from commit dd27cd1f84ecceb5fb287894373d4cb42131a03f)