Restart emission of all sub-particle systems at once.

Like in issue ~~#120~~ but for a whole graph sub-tree.

When working with complex emitters multiple emitter nodes are created and synced, however it is quite difficult to sync them if the model must be closed/opened after each sync change. What about a way to reset all emiters under a node? (eg: <r-click> -> reset emitters). This would help a lot when creating complex emission models.