lttng: Add an Eclipse builder for the doc plugin

lttng: Add an Eclipse builder for the doc plugin

Removed all committed .html and .xml documentation files
from the tree. Those can now be built either from Eclipse
(with right-click -> Run As -> Ant Build), or with Maven.

Splitted out the download-from-Eclipse-wiki task into a
separate target, which will only run manually.

Also did some small changes so that no user-editing of the
build file is required, except if you want to use a proxy
for the download.

Change-Id: I19d8d8e9c51aeda65dd26543ddade3a4d80e7803
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/9165
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
Tested-by: Bernd Hufmann <bhufmann@gmail.com>