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/lib.rs

@@ -72,6 +72,7 @@ use std::rc::Rc;
 
 use anyhow::Result;
 use dr_decode::{Metadata, PreviewSize};
+use slint::ComponentHandle as _;
 
 pub use develop::DevelopSession;
 
@@ -3436,7 +3437,7 @@ fn back_one_step(w: &AppWindow) -> bool {
         // Files named on the command line have no grid behind them — the same
         // condition the status strip uses to decide whether to offer the way
         // back at all.
-        has_grid: w.get_library_total() > 0,
+        has_grid: w.global::<Library>().get_library_total() > 0,
         scoped: w.get_collection_selected() != 0,
     };