Changed when the sidebar is opened.

Removed	the show() call	from showViewer().
Instead of this, I have manually added show() calls
to a few places. The result is this:

Don't automatically open the sidebar when
 * A new highlight is created
 * Heatmap is updated

But keep opening the sidebar when
 * Clicking on the heatmap
 * Clicking on a tab on the heatmap
 * Clicking on a highlight in the document
Host frame: created showFrame() and hideFrame() methods
Restored the original semantics of showViewer().

Introduced the updateViewer() method to do what showViewer() does,
except actually showing the sidebar.
Merge pull request #663 from hypothesis/649-sidebar-visibility-4

PR: Change when the sidebar is opened - v2