Encapsulate individual/group handling differences

This cleans up the PluginBridge significantly by getting rid of all
fields and handling that was only needed for connecting to plugin
groups. This was also the last thing I wanted to refactor before
releasing the plugin groups feature with yabridge 1.2.

src/wine-host/bridges/group.h

@@ -191,7 +191,7 @@ class GroupBridge {
      * Continuously read from a pipe and write the output to the log file. Used
      * with the IO streams captured by `stdout_redirect` and `stderr_redirect`.
      *
-     * TODO: Merge this with `PluginBridge::async_log_pipe_lines`
+     * TODO: Merge this with `HostProcess::async_log_pipe_lines`
      *
      * @param pipe The pipe to read from.
      * @param buffer The stream buffer to write to.