doc: start documenting Xwayland

This is a rough intro to what Xwayland is and does, with just one
implementation detail so far (Window identification).

I paid no attention to formatting details, those can be polished in
follow-ups. I just want the prose out.

I also just quickly whacked up the diagram, would be happy to see
someone replace it with a nicer one. I just didn't have time to learn
dot for now.

v2:
- typo fix
- rephrase "talking to hardware" as "driving the displays"
- mention circular dependency in intro
- add section to explain rootless and rootful modes
- remove paragraph about Xwayland protocol usage
- move TBD part to the end under a new section header

v3:
- use "advantage" and "disadvantage" instead of "pro" and "con"
- slight rewording on rootful mode and rootless mode paragraphs
- removed the paragraph about the lack of shell and special Wayland
  protocol extensions
- removed the commented out list of ideas to write

v4:
- typo fixes pointed out by Yong

Cc: Olivier Fourdan <ofourdan@redhat.com>
Cc: Jonas Ådahl <jadahl@gmail.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>

doc/publican/Makefile.am

@@ -24,9 +24,11 @@ publican_sources = \
 	$(srcdir)/sources/Preface.xml \
 	$(srcdir)/sources/Revision_History.xml \
 	$(srcdir)/sources/Protocol.xml \
+	$(srcdir)/sources/Xwayland.xml \
 	$(srcdir)/sources/Compositors.xml \
 	$(srcdir)/sources/images/icon.svg  \
 	$(srcdir)/sources/images/wayland.png \
+	$(srcdir)/sources/images/xwayland-architecture.png \
 	$(srcdir)/sources/Client.xml \
 	$(srcdir)/sources/Server.xml
 
@@ -43,7 +45,8 @@ css_sources = \
 
 img_sources = \
 	$(srcdir)/sources/images/icon.svg \
-	$(srcdir)/sources/images/wayland.png
+	$(srcdir)/sources/images/wayland.png \
+	$(srcdir)/sources/images/xwayland-architecture.png
 
 doxygen_img_sources := \
 	$(doxydir)/xml/wayland-architecture.png \