Fix up the editor manager API (internal+external)

Many of the methods in there have funny side effects. Others are inconsistent. Other API is public and looks like it shouldn't, or at least fixed up then.
To name a few things

• openEditorWithContents doesn't activate the editor, and doesn't take OpenEditorFlags
• openEditorsModel is exported + activateEditor(index, flags)
• activateEditor(view, editor, OpenEditorFlag::NoActivate) doesn't make sense, but is actually used (for the placeEditor call, which is a side effect)
• activateEditor(IEditor *) doesn't have any sensible semantic, since IEditors are not necessarily associated to a view (after split/unsplit)