do a better job of erasing the browser sidebars

do a better job of erasing the browser sidebars
ticket:127
the previous sidebar clearing code didn't remove the spacers between
the various widgets. This will do that (though for some reason
QLayout::removeItem wouldn't properly destroy my widgets, I had to use
QLayout;:removeWidget on them, which is why there are two loops in
SequenceBrowserSidebar::clear.

I also defined some typedefs in SequenceBrowserSidebar to abstract out
the exact type being used to store the sidebar detail widgets
(SequenceDescription) in case I want to switch to a boost::shared_ptr in case I want to switch to a boost::shared_ptr in case I want to switch to a boost::shared_ptr in case I want to switch to a boost::shared_ptr