Return napari viewer back to normal on close.

[adamltyson]

Now brainreg-segment is a plugin, it should leave the napari viewer alone, and things like disabling buttons should be removed.

[adamltyson]

Disabled in 78c0b2a. Keeping this issue open, to see if we can reset this on plugin close.

[adamltyson]

Also reset window title.

[adamltyson]

napari/napari#2670