~raof/mir/lots-of-unused-lambda-captures

Reimplement BroadcastingSessionEventSink using a similar "observer" idiom to that we use elsewhere.

Approved by Chris Halse Rogers, mir-ci-bot.

files renamed:
src/server/scene/session_event_sink.h => src/include/server/mir/scene/session_event_sink.h

added added

removed removed

* - copy-assignable

* - add():

* - copy-assignable

* - operator bool: returns whether this is a valid element

* - conversion to bool: indicates whether this is a valid element

* - remove(), remove_all():

* - copy-assignable

* - Element{}: default construction should create an invalid element

* - bool operator!=: inequality of elements

* - clear():

* - copy-assignable

* - Element{}: default construction should create an invalid element

* - Element{}: value initialization should create an invalid element

template<class Element>

{

ListItem() {}

RecursiveReadWriteMutex mutex;

Element element;

Element element{};

std::atomic<ListItem*> next{nullptr};

~ListItem() { delete next.load(); }