Hidden windows in the scratchpad failed to have their transparency
reset upon program exit. This is because they are kept separetly in
`ipc.get_tree.scratchpad()`, and not in `ipc.get_tree.workspaces()`.
Transparency gets reset when the script is terminated.
Added command line option to set transparency strength without changing
the script.
Added support for multiple displays.
Sway has ability to apply transparency to arbitrary windows. This script
wires up this functional to one of popular use-cases from
i3+<compositor_name>.
Signed-off-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>
