Plugin settings, installing new versions of plugin, getting and deleting plugin configurations

[hlg]

When a serializer plugin is installed from file and user settings do exist for that plugin, the default values are not applied. This makes only sense for options that did already exist earlier. For new options however, default values should be applied.

[hlg]

There are more issues associated with plugin (user) settings and their retrieval, modification and deletion. In particular:

• ServiceInterface.getSerializerByName fails after uninstall and new install of plugin, unless new user config created manually.
• PluginInterface.deletePluginConfiguration fails in most cases (exception: render engine plugin if auto-created user config).