dynamic module list

**Is your feature request related to a problem? Please describe.**
After installing a speech dispatcher module, one has to reload spd (and possibly screen readers ; orca doesn't need it) to get the module available

**Describe the solution you'd like**
speech dispatcher should notice modules dynamically and make them available

**Describe alternatives you've considered**
The user can reload spd, with SIGHUP it's then to the screen reader to get the list of modules again (orca does). It'd probably still be useful to make sure that that works in case spd crashes, but better not have to restart spd.

**Additional context**
The first thing needed for that would be to make the module list dynamic in the ssip protocol itself. And then spd could use e.g. inotify to watch the module configuration directories (or if not available, just probe from times to times), and load modules on the fly.

Start by reading the SSIP protocol and the current module discovery and loading paths; the issue does not name specific files. Check how SIGHUP refreshes modules, then investigate watching module configuration directories with inotify or periodic probing. Done means newly installed modules become available without restarting spd, while the SIGHUP path remains useful after crashes.

Written by the indexing model from the issue text.