layout: Always allow hiding the overview

When a fullscreen application is focused, toggling the overview via hot-corner is disabled, even when the overview is currently visible. This only makes sense, when the overview is hidden to not to disturb the behaviour of the fullscreen application, but leaves an inconsistency when the overview is visible since it should work there like when a non-fullscreen- application is focused.

So, always allow hiding the overview using the hot corner when the overview is visible.

Closes: #429 (closed)

I am not sure about the title of the commit message since everything else would exceed 72 characters.