Minor changes in default projects
All applet plugin follow standard plugin convention: __call__ return applet class.
To add widget in mainwindow and fill menus, just call "graft(oa_mainwindow=maiwnin)" method.

Move oalab.plugins interfaces to core.plugin
Real oalab plugins like applets, paradigms, remain in oalab.plugins
Add a splitable widget that tries to mimic the Blender UI.
All applet plugin follow standard plugin convention: __call__ return applet class.
To add widget in mainwindow and fill menus, just call "graft(mainwindow)" method.
Improve PluginManager.instances, get_applets
Move oalab.plugins interfaces to core.plugin
Real oalab plugins like applets, paradigms, remain in oalab.plugins
First draft to create a GUI inspired by Blender 3D GUI
Add applet icons
Move all plugins linked to plantgl in plantlab
Blender ui become a module instead of simple script
Fix --debug-plugins command line
First draft to migrate OpenAleaLab MainWindow to SplittableUi
Improve and simplify PanedMenu
Add global menu
Fix minor bugs and improve PluginInstanceManger error messages
Add test to oalab/example/test_project
First draft for Matplotlib widget based on openalea.oalab.plot2d compatible with new ui
First draft to add "show toolbar" mode for AppletContainer.
Add new Matplotlib Figure applet
Update test_project
Add icons
Improve SplittableUi
  - Tab icons are now horizontal also in East and West position
  - When a tab is closed, applet is destroyed and layout is now correct
  - layout now stores properties. That allow container and applets to save its states