Now that the QuickSettingsMenu supports child menus, it is time
to add menu support to quick toggles.
Do that by introducing a QuickSettingsItem parent class with a
construct-only :has-menu property, as that will allow including
menus in items that aren't following the standard icon+label
pattern of QuickToggle (yes, we'll have some of those).
A common parent class also allows us to control the type of
menu that is created. That's important, as we need another
custom menu type to
- constrain the menu's y position to place it underneath
the source actor
- use open/close animations taylored for this use case
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2393>
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