Change python plugin init order to have _gcompris first

To use the runit.sh script, it was needed to create a link
to gcompris python plugin. This was hard to understand why
it was not working when missing.

Now we init first the internal _gcompris python plugin
and the runit script add the directory to the python plugin
so that the link is no more needed.
removed deprecated include rsvg-cairo.h
simplified version of the submarine activity

The submarine activity is often seen in GCompris
as being too complex, even for adults. This new
version introduces a first level in which there
is no frigate, whales and crown to catch. The door
is opened by itself.

Also now the different control items in the schema
have tooltips to explain what they are about.