docs: Fix licence header

The library is LGPL, and the docs include the docs comments from the library
sources, so it's logical that the docs licence should be LGPL too. The docs
already said so in the text, but the licence header in the comments said GFDL.
Since I'm the only one who's contributed to the xml file itself, I just
changed this to LGPL, too.

https://bugzilla.gnome.org/show_bug.cgi?id=728795
Prepare version 0.37.0
all: Reorganise headers
build: Work around include directory issue

The correct way to include vte.h is <vte/vte.h> but the source directoy
is named src/ not vte/ so this doesn't work while building vte itself,
and it breaks when building the vala test application.

Make vte/vte.h symlink to src/vte.h, and dist that. Ugly, but it works™.
Post release version bump