💥 Encapsulate and rework all socket logic

This is a pretty huge change that will be important for being able to
handle nested or mutually recursive `dispatch()` and `audioMaster()`
calls. This sadly all had to be done in a single commit, so here's a
summary:

- `src/common/sockets.h:Sockets` contains all sockets on both the plugin
  and the Wine host side, and is used to both listen on and connect to
  the sockets.
- Sockets and other temporary files respect `$XDG_RUNTIME_DIR` instead
  of being dumped in `/tmp`.
- All sockets now have a unique endpoint in
  `/run/user/<uid>/yabridge-<plugin_name>-<random_id>/`. This is
  important for when we want to have multiple socket connections for
  handling `dispatch()` and `audioMaster()`.
- Because of the above, we no longer clean up the socket endpoint files
  after the connection gets established during initialization. Instead
  we'll remove the socket base directory when shutting down.

src/common/serialization.cpp

@@ -110,5 +110,6 @@ AEffect& update_aeffect(AEffect& plugin, const AEffect& updated_plugin) {
 }
 
 bool GroupRequest::operator==(const GroupRequest& rhs) const {
-    return plugin_path == rhs.plugin_path && socket_path == rhs.socket_path;
+    return plugin_path == rhs.plugin_path &&
+           endpoint_base_dir == rhs.endpoint_base_dir;
 }