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/collections_ui/press.rs

@@ -6,8 +6,9 @@ use std::collections::BTreeSet;
 use std::rc::Rc;
 
 use dr_types::ImageId;
+use slint::ComponentHandle as _;
 
-use crate::AppWindow;
+use crate::{AppWindow, Library};
 
 use super::controller::CollectionsController;
 use super::tree_sync::sync_selection;
@@ -292,7 +293,7 @@ pub fn cancel_press(window: &AppWindow, ctl: &Rc<CollectionsController>, ids: &[
     // has to put it back down, or the ring stays open around a cell nobody is
     // touching and the grid stays frozen with it.
     ctl.pending_toggle.set(None);
-    window.set_library_held_row(-1);
+    window.global::<Library>().set_library_held_row(-1);
 
     let Some(undo) = ctl.press_undo.borrow_mut().take() else {
         return;