[IZPACK-1106] Disable Next button on PacksPanel if no visible pack is selected

Currently it is possible to unselect all packs in the PacksPanel and switching to the next panel. This is not intuitive and makes no sense from the user's point of view, even if there are hidden panels.

There should be at least one visible panel selected on the packPanel, either by the user or a required one, to show the user that something will be installed when he presses Next.

Disable the Next button as long as there is no pack selected.

Added also some smaller code cleanups concerning the existing PacksPanel.