SimpleSpawner should have "spawning" property visible in Inspector

[upost]

Currently, a SimpleSpawner does nothing until explicitely activated by the "start spawning" block. I think this is not intuitive.

Suggestion for Improvement:
Export the "spawning" property to make it a visible checkbox in the explorer (unchecked by default to remain compatible).

Reason:
It is counter-intuitive that a sole SimpleSpawner in a scene does nothing, because other comparable nodes (i.e. GPUParticles) are enabled by default and have a visible property representing this state. While a game maybe wants to activate the spawner not on starting the scene but only somewhat later, for beginner users (like my pupils ;-) ) it is not easy to understand that they need a block to activate the node, because the inspector does not show an activation property. Having a checkbox makes it easier to understand how to activate or deactivate the spawner. For scenes which do not need to change this property, then there's no need to create block code at all which makes the setup lighter and easier to understand.