windowPreview: disconnect from window title changes on actor destroy

[3v1n0]

When a popup is destroyed, the window might still emit signals of
title changed, causing crashes in gnome-shell

We're getting multiple crash reports about this in both Ubuntu (https://pad.lv/1714989) and upstream GNOME (https://bugzilla.gnome.org/show_bug.cgi?id=788931)

[jderose9]

Thank you, this is very appreciated!

[3v1n0]

Thanks for merging. Please also backport this to the version in extensions.gnome.org as soon as you can, so we can filter out similar crashes we're getting also from people without this.

:)

[jderose9]

@3v1n0 I just submitted the newest version for review (though of course it doesn't necessarily mean every user will upgrade). Thanks!

https://extensions.gnome.org/review/7552