Implement animation/player behavior for QQuickItemGenerator

VectorImage can't depend on QtQuick.Timeline for its own animation
composition — the license would be more restrictive, and
quickvectorimage doesn't want a dependency on another module either
way. qtlottie shares Timeline's license tier, letting this commit
build a concrete provider on Timeline semantics here.

LayerItem also implements the transform-source interface, so that a
layer can act as the transform another layer refers to.

Timelines are chosen by the reference the generator data carries so that
a node's own transform binds to the correct frame counter. Resolving by
name also lets the frame counter mapper state the timeline it maps from.

Task-number: QTBUG-147709
Change-Id: I2833e374a3facff8c471a312128c409e7a977491
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>