Add initial image viewer

ui/types.slint

@@ -0,0 +1,28 @@
+export enum PreviewKind {
+    None,
+    Thumbhash,
+    Thumbnail,
+}
+
+export struct ImagePreview {
+    asset_id: string,
+    image: image,
+    kind: PreviewKind,
+}
+
+export enum Visibility {
+    Hidden,
+    NearView,
+    InView,
+}
+
+export struct ImageBucket {
+    key: string,
+    title: string,
+    count: int,
+    previews: [ImagePreview],
+    y: length,
+    height: length,
+    visibility: Visibility,
+}
+