Fix flakes8 errors

Fix some flake8 errors on core
Remove obsolete ProjectWidget
OpenAleaLab can run without LPy, openalea-trunk, ...
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)
All applet plugin now follow same API
All applet class can be instantiated with zero parameters
get_applet service is now based on PluginManager
Add a splitable widget that tries to mimic the Blender UI.
PEP8
Example to test blender ui
First draft to support OpenAleaLab applets