Move the library's routes and status onto its own Slint global

The scan/opening/status/error lines, offline mode and its retry, pinning a
collection offline, the sync and thumbnail-sweep state, and the callbacks
that route the grid to a rescan, a sync, another library, a panorama merge
or an export — the last of what AppWindow still carried under the
library- prefix — move onto the `Library` global started earlier on this
branch. Rust reaches them through window.global::<Library>() rather than
window.set_/get_/on_/invoke_ on the root.

library-visible is the one name that stays: it is computed from
active-page and active-view, the shell's own routing state, which a
global cannot read. AppWindow now declares no other library- property or
callback.

ui/dr-ui/src/library_ui/timeline.rs

@@ -657,7 +657,7 @@ pub(super) fn write_place(window: &AppWindow, ctl: &Rc<LibraryController>) {
 fn current_place(window: &AppWindow, ctl: &Rc<LibraryController>) -> Option<dr_types::Place> {
     use dr_types::{Place, PlaceScope, Screen};
 
-    if window.get_active_view() == View::Launch || !window.get_library_open() {
+    if window.get_active_view() == View::Launch || !window.global::<Library>().get_library_open() {
         return None;
     }