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.

New default window appearance (without !401 (merged) applied):

cc @teams/vdg