Button "Toggle the search tree" from left toolbar of Symbol Library
Editor does not show actual state of visibility of the search tree
panel. State of this button (checked or unchecked) changes on clicking
only. Toggling panel visibility from menu does not make effect on tool
state.
Applied changes makes tool state (checkd/unchecked) synced with panel
state (shown/hidden).
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