CustomLv2SearchPlugins: add search filter 'presets' and 'uri <plugin_uri>'
CustomLv2Host: check custom UI of a plugin

We currently just check if the binary exist. The next step is to load
the binary...
CustomLv2Host: check scale point of a plugin
CustomLv2Host: check plugin class
CustomLv2Host: check presets

* can list the presets available for a plugin
* still can't use a preset to update value of ports...