Fix bugs in lpy control imports
Move oalab.plugins interfaces to core.plugin
Real oalab plugins like applets, paradigms, remain in oalab.plugins
First draft for plugin manager
All applet plugin follow standard plugin convention: __call__ return applet class.
To add widget in mainwindow and fill menus, just call "graft(mainwindow)" method.
PluginManager is now a singleton.
Singleton instance attribute is now private (use call to get it)