This patch creates the `ExpandableListItem`, a re-usable PlasmaExtras item that can
be used for the views in various Plasma applet pop-ups, such as Vaults, Printers,
Bluetooth, Networks, and Device Notifier. This way those applets can share more code and
not have to implement this paradigm themselves in five different ways, as they currently
do.
All of these applets currently use slightly different visual styles. For example the
network applet uses a pushbutton with no icon as its "default action" button, while
other applets use icons-only toolbuttons. I tried my best to create a component that's
flexible but also consistent, so various applets that adopt it will see minor visual
changes as a result. Hopefully this is acceptable.
Closes T12812