slot: finish moving all other private data to private struct

Now that we got rid of nautilus-window-manage-views, move all the
private data in NautilusWindowSlot outside of the public struct.
Merge commit 'cbbf9c88548f2295921bfe9fbf30ea0ca59d1b6f' into nemo_38_merge

Conflicts:
	src/nemo-desktop-window.c
	src/nemo-toolbar.c
	src/nemo-window-menus.c
	src/nemo-window-private.h
	src/nemo-window-slot.c
	src/nemo-window-slot.h
	src/nemo-window.c
	src/nemo-window.h