Properly set emitting when particles restart

[clayjohn]

Fixes: #27146

Partially addresses: #26042

For GPU particles, an issue was that when restart() was called, the code waited until the next frame to restart the particles (and set emitting) to true. This resulted in a situation where emitting could be set to false and then the driver would set it back to true. When that issue was fixed, the fix caused restart() to no longer set emitting to true. Now, emitting is set to true at the same time restart() is called, so the previous bug should not arise, and restart() will properly restart the particle system.

The reason #26042 is only partially addressed, is that in that issue, the user wants the particle system to restart when emitting is set to true. However, there are valid use cases where one may want to set emitting on and off without it restarting. So a proper fix should not restart when emitting is set to true (as I suggested in the thread). Instead, in the AnimationPlayer, the user should set emitting to false when they want it to stop, and then call restart() when they want to restart the system. However, the AnimationPlayer won't call methods when running in editor. So the workflow is still not ideal. To partially address that, I added a restart button to the particle editor.

Code graciously donated by Gamblify.

CC @jesperkondrup, @akien-mga

[JFonS]

Looks good to me.

[akien-mga]

Thanks!