Panel: Visibility hidden for closed panels. Fixes #5652.

This prevents focus on links in closed panels when using the tab key.
Core: Only set a default min-height for the page. Fixes #5755

resetActivePageHeight() only overrides the page min-height. Setting this for ui-mobile causes issues on small screens and according to the commit message (ead0baf) we don't really need it.
Panel: Call positionPanel() before we start opening the panel.

This prevents a vertical scrollbar on the page div when opening a long panel on a short page.
Demos: Removed panel workaround. Problem has been fixed in framework.