switcherPopup: Fix switch-group mouse click regression

switcherPopup: Fix switch-group mouse click regression

The LongPressGesture 'may-recognize' callback in commit 314137c8c was
checking if clicks were inside '_switcherList', but thumbnails shown by
switch-group are siblings of _switcherList, not children.

This caused the gesture to incorrectly identify thumbnail clicks as
'outside' clicks, closing the popup before the click could complete.

Fix by adding a _isActorOutside() helper method that subclasses can
override to check additional containers. AppSwitcherPopup now properly
checks both _switcherList and _thumbnails.

Tested on Ubuntu 25.10 with GNOME Shell 49.0. Confirmed:
- switch-group mouse clicks now work
- switch-applications and switch-windows unaffected
- clicking outside the popup still closes it correctly
- no regressions introduced

Fixes: 314137c8c9 ("switcherPopup: Move to a ClutterGesture for closing the popup")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8812
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3959>