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/drag.rs

@@ -10,7 +10,7 @@ use dr_catalog::Catalog;
 use dr_types::CollectionId;
 use slint::ComponentHandle;
 
-use crate::AppWindow;
+use crate::{AppWindow, Library};
 
 use super::controller::CollectionsController;
 use super::tree_sync::refresh_tree;
@@ -189,11 +189,11 @@ pub(super) fn arm_hold(window: &AppWindow, ctl: &Rc<CollectionsController>, row:
             // This half happens whether or not selection mode was already on —
             // it is the half a *drag* needs, and a drag out of a selection of
             // forty starts in a mode that is already on.
-            w.set_library_held_row(row);
+            w.global::<Library>().set_library_held_row(row);
 
             if !ctl_cb.select_mode.get() {
                 ctl_cb.select_mode.set(true);
-                w.set_library_select_mode(true);
+                w.global::<Library>().set_library_select_mode(true);
             }
 
             // The release that follows this hold must not also open the image: