view: only focus topmost view if unmapped view was focused

view: only focus topmost view if unmapped view was focused

The unmap() handlers should only call desktop_focus_topmost_view() if
the unmapped view was the focused view. Unmapping a view that was not
focused should not change the focus.

I expect this rarely had any effect in practice; it would only matter in
a focus-follows-mouse config where some view other than the one on top
was focused. But it still seems better to fix.

Rather than repeating the logic in two places, create a small
view_impl_unmap() helper. Perhaps more common "unmap" logic could be
moved there in future.