implement a runtime frame control

Timeline has 'currentFrame' property.

When reading it, it shows current rendered frame number.

When writing a value to it, scene moved to the specified frame.

However, it cannot be controlled at runtime and an initial value is ignored when starting the animation.

In the attached qml,

initially an animated square will be placed at around (40, 40).

A red square in the left-bottom will make the animation start/stop when clicking on it.

A blue square in the right-bottom will try to move the target to (100, 100).

Setting currentFrame cannot affect animation itself, only temporally evaluate the temporal values.

And the patch in gerrit can make it possible to adjust at runtime.