Also changes Save Library and Save Library As to work off the visible selection if there is one. Also implements enablement for Save All Libraries. Fixes: lp:1764081 * https://bugs.launchpad.net/kicad/+bug/1764081
Fixes: lp:1761058 * https://bugs.launchpad.net/kicad/+bug/1761058
Fixes: lp:1748555 * https://bugs.launchpad.net/kicad/+bug/1748555