Move the library grid's cells and selection onto their own Slint global

AppWindow carried the grid's loaded window of cells, the keyboard cursor,
drag and drop, the held-row long-press state, columns and cell size, the
scroll and viewport bookkeeping, the photo roll's pick and centre-request,
and the local-only/reorder/collection-filing gestures that act on a
selection, as properties and callbacks on the root component. That state now
lives in the `Library` global declared in library.slint, next to the structs
(LibraryCell, TimelineBar, KeywordRow, PersonChip) it and the grid's other
components already share; Rust reaches it through
window.global::<Library>() instead of window.set_/get_/on_/invoke_ on the
root, the same change collections.slint's `Collections` global made for the
sidebar.

library-visible stays on AppWindow: it is computed from active-page and
active-view, the shell's own routing state, which a global cannot read.
Everything else still prefixed library- — the timeline, the filter bar,
ratings and flags, keywording, and the routes and status lines — stays on
the window for now and moves in the commits that follow.

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

@@ -18,7 +18,7 @@ use dr_types::FormatFilter;
 use slint::ComponentHandle;
 
 use crate::library::{self, ScanMessage};
-use crate::{AppWindow, View};
+use crate::{AppWindow, Library, View};
 
 use super::controller::{stop, LibraryController};
 use super::offline::refresh_offline;
@@ -677,11 +677,12 @@ pub(super) fn schedule_reload(window: &AppWindow, ctl: &Rc<LibraryController>) {
         // change to `columns` moves every one of them and a viewport left
         // where it was would be pointing at rows the window no longer covers.
         let window_size = *ctl_cb.window.borrow();
-        let total = w.get_library_total().max(0) as usize;
+        let total = w.global::<Library>().get_library_total().max(0) as usize;
         *ctl_cb.offset.borrow_mut() = window_start(anchor, window_size, total);
         load_window(&w, &ctl_cb);
-        w.set_library_scroll_to(anchor as i32);
-        w.set_library_scroll_token(w.get_library_scroll_token() + 1);
+        w.global::<Library>().set_library_scroll_to(anchor as i32);
+        w.global::<Library>()
+            .set_library_scroll_token(w.global::<Library>().get_library_scroll_token() + 1);
     });
 
     // Replacing the slot drops the previous timer, which is what makes this