Allow plugin action invokation via API

[smlpt]

In addition to being able to access plugin actions programmatically via mastodon.plugins.getPluginActions().actionMap.get("foo"), it would be nice to also be able to invoke these actions or even attach a callback to them in case they are longer running actions, to get a signal once the calculation has finished.

My current workaround is:
myAction?.actionPerformed(ActionEvent(pluginActions, 0, null))