testing/lsp-plugins: add -xdg subpackage and move xdg integration to it

Some desktop environments (like GNOME) don't play nicely with the XDG integration, which causes the application menu to get spammed with individual icons for standalone plugins. They aren't required for most usecases, and as such can be moved into a separate subpackage.

cc @psykose; xdg support was enabled alongside an upgrade without a given rationale, was there any particular reason for why it was enabled, i.e. something that should be tested to make sure this doesn't break anything? easyeffects seems to work fine without the xdg bits