[alt-tab] Show windows from all workspaces. Highlight "urgent" windows.
[Alt-Tab] No gradual fade-in of preview backdrop.
[Alt-Tab] Respond to Home and End keys. Remove obsolete key bindings.
[Alt-Tab] Avoid duplicates of windows displayed on all workspaces.
[Alt-Tab] Scroll icon list earlier when we are approaching the edge.

Before, we only scrolled when we reached the egde, now  we
scroll when when we are about four items from either egde.
The new behavior should only be visible when we have a lot of
windows open and the icon list cannot show all windows without
scrolling.
[Alt-Tab] When activated on an empty workspace, don't default-select any window at all.
[Alt-Tab] Enable multi-line title labels.
[Alt-Tab] Avoid different icon heights when scrolling through the icons.

Since the selected icon may have a different style than the rest, it's
bad to have that icon be the model for all icons.
[Alt-Tab] Size allocation is smoother if 'squareItems' is false.