I3 wm

Adds formulas for the i3 tiling window manager and the startup-notification library it requires.
```
$ >t.h
$ clang -x c-header t.h -o t.h.pch
$ echo $?
$ clang -x c-header t.h -o t.h.pch -Wl,-v
clang: error: cannot specify -o when generating multiple output files
```