Update extensions to newer version and to the URLs from openvsx instead of
github, since openvsx prevent maintainers to modify releases.

add vscode-references-view (
https://open-vsx.org/extension/ms-vscode/references-view ) it's installed
in default Theia and looks good.

Some java extensions were not on openvsx, they were removed or updated.
Newer java extension seem to associate .cfg as "properties" files, so force a
mapping to make them open as zc-buildout.

python was also updated, but not to latest version which does not work on
Theia.