Add presel modifier

Allows selecting windows based on whether or not they are preseled.
This is useful for duplicating the old shift-can-be-transplant
behavior.

    # Usage: squish DIR
    #  - If there is a preseled window in the given direction, transplant
    #    into its node.
    #  - If the focused window is preseled, transplant the window in the
    #    given direction into it.
    #  - Otherwise, swap.

    squish() {
        {
            dir=$1
            if bspc query -W -w ${dir}.presel; then
                bspc window -w ${dir}
            elif bspc query -W -w focused.presel; then
                bspc window ${dir} -w focused
            else
                bspc window -s ${dir}
            fi
        } >/dev/null
    }
Document presel modifier.