plugins/fadingpopups: Blacklist spectacle popup menus from fading effect

Spectacle can trigger a screenshot from a popup menu. This menu needs to get hidden immediately with no effects, to avoid appearing on the screenshot.

Test plan

Launch a region/full-screen screenshot from this menu in spectacle

spectacle_menu

The menu should disappear immediately and not appear on the screenshot

Screen recordings

BEFORE AFTER
spectacle-fadingmenu-before spectacle-fadingmenu-after

Bugs fixed

BUG: 505803
FIXED-IN: 6.4.2

Edited by Ismael Asensio

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