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.

meson.build

@@ -65,6 +65,7 @@ shared_library(
     'src/common/logging.cpp',
     'src/common/serialization.cpp',
     'src/plugin/configuration.cpp',
+    'src/plugin/host-process.cpp',
     'src/plugin/plugin.cpp',
     'src/plugin/plugin-bridge.cpp',
     'src/plugin/utils.cpp',