Replace keylaunch by xbindkeys

I see a few reasons to replace `keylaunch` by `xbindkeys`:
 - `xbindkeys` can use a GUILE configuration file, so we can apply bindings in specific cases only, in our case we can have a single configuration file for X11, Xephyr and host X server.
 - `keylaunch` does not have a proper package in Arch, Fedora, Suse (Ubuntu, Debian, Gentoo support it). `xbindkeys` is packaged by all these distributions.
 - `xbindkeys` allows to trigger events on mouse clicks (e.g. Ctrl+click can do something).

This branch does the following:
 - Install and run xbindkeys on all x11 targets (x11 or xephyr).
 - Map `ratpoison` bindings if we are using Xephyr, or `ratpoison`-like on X11 (both host X11 and chroot X11)
 - Map brightness keys (chroot X11/Xephyr only).
 - Map Alt+click to the middle button. If find it very hard to do the 3-fingers click on the Samsung ARM (half of the time I get a right click). As far as I know Alt+click is not often used (unlike Ctrl+click and Shift+click), and Chrome OS maps it to right click. That could still break some applications (I think 3D apps like blender may use Alt+click for some purpose...), could we have some global crouton configuration file so that the user can enable/disable this kind of stuff?

Volume can also be mapped once my other branch is cleaned up. ([chroagh/volume](https://github.com/drinkcat/chroagh/commits/volume))