river/tags: Allow river tags to be styled based on monitor focus

[lilly-lizard]

Adds the output class to the river tags module when a river output (i.e. monitor) is focused. This allows the tags to be styled differently depending on wherever the monitor is focused or not so the user can identify which monitor is in focus.

Example style.css

#tags button {
        color: @grey;
        padding-left: 30px;
        padding-right: 30px;
}
#tags button.occupied {
        color: @foreground;
        transition: none;
}
/* tag is focused */
#tags button.focused {
        color: @foreground;
        background: @shaded;
        border-bottom: 2px solid transparent;
        border-top: 2px solid transparent;
}
/* tag is focused and output is focused */
#tags button.focused.output {
        border-bottom: 2px solid @underline;
        border-top: 2px solid transparent;
}