feat: Centralise selection state to share between Tile/Grid view

Description

Move TileView out of UI.tsx into a new file, to match TableView
Move the image selection state into UI, and then pass the value and an update function into both views, so they're in sync and work with our external sort and filter.
- It's now a Set rather than an array as then me don't need ot do as much management of duplicates etc
- It's a reducer, if Ui.tsx was functional, it's an easy swap to useReducer hook.
assigneesLenght -> assigneesLength everywhere 😀
Add filterShow to MetaItems to replace selected as selected means "highlighted" throughout now
thumbnailWidth. thumbnailHeight are now just one state value as thumbnails are square
TileView and TableView only receive the filtered and sorted metadata now, I don't think they care about the full set? (maybe size of full set to show x of y items shown in future?)