webadmin: Fix small action button regression

As part of the action button scrolling feature, action buttons that
were supposed to be hidden were revealed. This patch tries to fix
this, by only considering action buttons that were visible to begin
with.

I also found no reason to keep invoke setVisible(true) on buttons that
were already rendered visible, nor to keep summing the widths of
hidden buttons (which should be zero anyway, as I understand from the
fact that buttons had been made visible to measure their widths).

Change-Id: I3a977d2dc2bf929c9f0fb5853ba667cfe3728a37
Signed-off-by: Lior Vernia <lvernia@redhat.com>