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/export.rs

@@ -67,8 +67,9 @@ use dr_export::{Encoded, NameContext};
 use dr_sync::RemotePath;
 use dr_sync::{Account, Connection};
 use dr_types::{ExportSettings, ExportTarget};
+use slint::ComponentHandle as _;
 
-use crate::AppWindow;
+use crate::{AppWindow, Library};
 
 /// Where exports wait for a server that is not there yet.
 ///
@@ -1131,7 +1132,7 @@ fn summarise(
 fn report(window: &AppWindow, reporting: Reporting, text: &str) {
     match reporting {
         Reporting::Develop => window.set_export_status(text.into()),
-        Reporting::Library => window.set_library_status(text.into()),
+        Reporting::Library => window.global::<Library>().set_library_status(text.into()),
     }
 }
 
@@ -1140,7 +1141,7 @@ fn settle(window: &AppWindow, reporting: Reporting, text: &str) {
     report(window, reporting, text);
     match reporting {
         Reporting::Develop => window.set_export_busy(false),
-        Reporting::Library => window.set_library_exporting(false),
+        Reporting::Library => window.global::<Library>().set_library_exporting(false),
     }
 }