Tile X11 windows vertically in a box.

Useful with https://github.com/tleino/mxswm

Example
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# Start programs you want
$ xclock &
$ xbiff &
$ xload &

# Figure their IDs
$ xwininfo -root -tree | grep xclock
$ xwininfo -root -tree | grep xbiff
$ xwininfo -root -tree | grep xload

# Put them to xvbox
$ xvbox 0x80000c 0x40000d 0x50000c