A function to add global handlers to the client (e.g. for didReceiveGlobalSettings event)

[StefanZoerner]

For the getGlobalSettings function of Stream Deck it seems to be necessary to use a handler which is not specific to an action. But for the plugin as a whole.

The Client type already has a data structure (handlers) for general handlers (file client.go). And the handlers are called (if any). I have added a function to add a "global" handler. Without I was not able to work with didReceiveGlobalSettings events in my plugin.

I have added a go.mod file for the Go module system as well.

Thanks for your work, Sam

[FlowingSPDG]

Hello, I am also curious this
however I made new fork with some additions(includes this) so you could use it until Sam merges this PR: https://github.com/FlowingSPDG/streamdeck