Cycle backwards with Shift

[git-f0x]

This should fix #173 and pop-os/cosmic-comp#727.
The reason for a new message is that it could be potentially used in the future to e.g. undo the last Tab autocomplete.
It's possibly not the best way to do this, but it seems to work.

Unrelated, it seems the reason for the extra vertical padding between buttons and dividers is the divider. Changing it to horizontal_rule eliminates the extra vertical padding. Not sure if it should be changed to horizontal rule, or if the divider in libcosmic should be somehow changed.

[pikachunakapika]

Works pretty good. In the near future this also needs to be covered in shortcuts config:

[bbb651]

This only fixes half of the problem, it won't work unless the launcher is already open (hence why it's not actually a shortcut), meaning you cannot press alt + shift + tab to open the launcher and already be focused on the last window in the list (which would be useful when listing windows by focus order is implemented).

Actually this currently doesn't work with alt + tab either because currently this logic is wrong:

Message::AltTab => {
    if self.alt_tab {
        self.focus_next();
    } else {
        self.alt_tab = true;
    }
}

It should always focus_next() even on the first time.

I tried to implement it before and was a bit confused as to make optional boolean arguments in clap, I think I was looking for ArgAction::SetTrue.

[git-f0x]

Yeah, this is kind of janky as is. I attempted to make some changes to cosmic-comp and cosmic-settings-daemon to maybe somehow make it work with a subcommand (cosmic-launcher shift-alt-tab), but I only managed to break every shortcut using the Super or Alt Keys.
I'll make this a draft for a while until I maybe figure out a way, and close this if not.

[git-f0x]

I'll close this in favor of #183.