Presel modifier

Allows selecting windows based on whether or not they are preseled. This is useful for duplicating the old shift-can-be-transplant behaviour (among other things like sending the focused window to the last focused preseled window).

```sh
# 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
}
```
Should fix: https://bbs.archlinux.org/viewtopic.php?pid=1299828#p1299828