This is more of a workaround than a fix. Simply by forcing the
URI to be sortable (VST-user presets are sorted at the end).
This eventually needs a deeper cleanup and API consolidation
PluginInfo::get_presets(bool) returns an ordered std::vector.
However this API is only used in a few cases where the plugin is
not instantiated. PluginInfo::get_presets(bool).
The problem is Plugin::get_presets(). The method returns
information that was previously cached by Plugin::find_presets() in
std::map<URI, ...>
Set the AEffect* plugin pointer before calling effOpen.
effOpen may call back into the host (masterCallback) and invoke
actions which depend on _plugin (eg. to call back into the plugin again)
