Object editor improvements [Test, don't merge it]

Here is some improvements to the object editor (and the object selection dialog).
If you can test it and tell me what do you think of these improvements ? (not finished yet, it's just some ideas)

List of improvements : 
 * Separate groups from objects
 * Global objects/groups are shown in bold
 * Edit groups directly from the object editor : you can unfold a group to see the objects inside
 * Drag an object into a group to add it to the group
 * Quick group creation : Drag an object on "Groups" (will show "Create a new group") and GDevelop will create a new group with this object inside.