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/bursts.rs

@@ -45,9 +45,9 @@ use dr_catalog::bursts::{self, Rules, Signature};
 use dr_catalog::Catalog;
 use dr_thumbs::{ThumbSize, ThumbStore};
 use dr_types::ImageId;
-use slint::Model as _;
+use slint::{ComponentHandle as _, Model as _};
 
-use crate::AppWindow;
+use crate::{AppWindow, Library};
 
 /// How many signatures are written per transaction.
 ///
@@ -364,7 +364,7 @@ pub fn sync_badges(window: &AppWindow, catalog: &Catalog, ids: &[ImageId]) {
         }
     };
 
-    let model = window.get_library_cells();
+    let model = window.global::<Library>().get_library_cells();
     for (row, id) in ids.iter().enumerate() {
         let (count, expanded, representative) = match found.get(id) {
             Some(m) => (m.size as i32, m.expanded, m.representative),