QQuick3DParticleSceneShape: port m_data from QList to std::vector

The container is never copied, so we don't need the overhead of CoW
and the detach attempt code bloat that goes with it.

Only one operation needed to be modified: append() → push_back(). As a
drive-by, add an opportunistic std::move() (at least the two
Connection objects are much more efficiently moved than copied).

This is in preparation of introducing non-const iteration over the
list in order to not pass const QMetaObject::Connection objects to
disconnect(), which is going to be deprecated (and removed, in strict
mode).

As a drive-by, group the header includes by module (just inserting
blank lines between them).

Amends 18577ba0b6ca9e9aa6cc69c8fa010de0aae13c41 (6.11).

Pick-to: 6.12 6.11
Change-Id: I40df321d8fdf14178c79a0944364f309edeca9fe
Reviewed-by: Christian Strømme <christian.stromme@qt.io>