This commit makes the "Show Sidebar" button that's on the toolbar by
default use the new `sidebar-expand` icon, if present. If not, it falls
back to the old `view-sidetree` icon for compatibility's sake.

We also make thie button icons-only by default, as the new icon is
clearer than the old one. This saves some horizontal space in the
defualt toolbar, which is now quite wide.