Implement Plugin::get_editor() for PluginInstance

[Boscop]

If someone is interested in doing this, that'd be great (I'm quite busy atm).
It only requires creating a PluginInstanceEditor struct and storing it in the PluginInstance struct, it can be filled with values gotten from sending the editor OpCodes to the instance, EditorGetRect etc. Basically look at for which OpCodes interfaces::dispatch() calls plugin.get_editor() and "invert" that direction to construct a PluginInstanceEditor.