Restore script DBus API

The Script class has a DBus API we want to export. At some point this
has got broken and the run method is not exported.

It's a bit messy with script subclasses also having other invokables
that we want to export to scripts, so an adaptor is used to keep things
separated.

The interface name has technically changed, but KWin was doing something
weird and using the same interface names for the manager. Fortunately
calling Plasma code doesn't specify an interface so this still works.