Use icons-only "Show Sidebar" Button with new icon, if available
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.