Log Message:
wm: Add herbstluftwm.

Based on work by voidpin and benny in wip, with fixes.

herbstluftwm is a manual tiling window manager for X11 using Xlib and Glib.

Its main features can be described with:

  - The layout is based on splitting frames into subframes which can be
    split again or can be filled with windows (similar to i3 or musca)

  - Tags (or workspaces, or virtual desktops) can be added/removed at
    runtime.  Each tag contains an own layout

  - Exactly one tag is viewed on each monitor.  The tags are monitor
    independent (similar to xmonad)

  - It is configured at runtime via IPC calls from herbstclient.  So the
    configuration file is just a script which is run on startup.  (Similar
    to wmii or musca)