windowManager: Always check dimming when an attached modal opens/closes

windowManager: Always check dimming when an attached modal opens/closes

We skip window animations while the overview is shown (and the window
group is hidden) to avoid unnecessary work. However when an attached
modal dialog is opened or closed, this involves checking whether the
parent window should be dimmed - skipping that test means that we can
simply fail to dim or undim a window altogether, so do that check
unconditionally.

https://bugzilla.gnome.org/show_bug.cgi?id=762475