Extrusion bracket examples rotated for clearer view and made optional.
Added missing description for scs_bearing_blocks.
Extrusion no longer shows centre parameter but does show no default cornerHole.
Moved screw_insert() from inserts.scad to screw.scad.
Changed most includes of inserts.scad to use insert.scad.
Removed linear_bearings, iecs, rockers and microswitches from lib.scad as
already included by other vitamins.