Sort VST factory presets

Sort VST factory presets

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, ...>