Separate function of left and right panels

[sferot]

As proposed in the mailing list we implemented some modifications to the three panels view. We separated the functionalities : We add every kind of items with the left panel, and all the configuration is done with the right panel. We also add a way to reduce/close the right panel. It will stay closed until voluntarily reopened. You can see the result in the following video : https://webmshare.com/ayKMQ

[nakomis]

@sferot Could you rebase this against master please, it would be nice to get this merged