no animation when destroying thumbnail of destroyed window
scroll arrow should only be shown if there are more items to see or an edge item is barely visible

I have often scrolled to the end or the beginning, only to find that the scroll
arrow was shown without there being any additional item past the edge.
avoid determinining scroll capabilities too early, before the switcher has entered the stage
let F5 toggle between multi-workspace mode and single-workspace mode, freeing up plus and minus for other uses