tabbox: Make thumbnail_grid switcher a popup

With this, the thumbnail grid task switcher window won't be viewed as a toplevel window anymore and the scale effect won't attempt to animate it, which doesn't look good currently.

Given the input semantics of the task switcher, it might be preferred to create a non-grabbing popup and then let the tabbox input filter do its thing, but Qt has no matching non-grabbing popup flag, only ToolTip, which would work in this case but it would look odd.