desktop: harden window stacking order while window switching

desktop: harden window stacking order while window switching

This commit moves the check against server->input_mode from the callers
of desktop_focus_view() into desktop_focus_view() itself. This
eliminates code duplications and makes it harder to mess up the window
stacking order while window switching.

I also added the same check in view_minimize() so that minimize requests
from panels never messes up the window stacking order (I think only this
should be described in the release note).