Add keybindings for window tiling

These keybindings were written by Julien Lavergne <gilir@ubuntu.com>
and included in the GPL lubuntu-default-settings Ubuntu package. The
only contributions of mine are cleaning up the whitespace and changing
<height>97%</height> to <height>100%</height>.

I intend to get these keybindings into LXDE (see [1] and [2]) but I wanted to give the Openbox developers an opportunity to include them upstream.

[1] https://sourceforge.net/p/lxde/feature-requests/191/#5372
[2] http://git.lxde.org/gitweb/?p=lxde/lxde-common.git;a=blob;f=openbox/rc.xml.in;h=8db93bfee711da94a37ca78088093f6858f8a8ca;hb=HEAD