Persisting connection settings

[ErikBooijMB]

Hi!

First off, your SMPP GUI is awesome, massively appreciated, since there are very few other options available on macOS.

I was wondering if you have considered persisting connection settings across restarts, or better yet, being able to define a list of connection settings (YAML, JSON, or even XML), and then having a dropdown list to pick from a connection config from that file.

Normally I'd take a stab at it, and ask you to consider a PR, but I'm an absolute Java noob, so here I am, pleading to you with puppy eyes.

Fully understand if you're not taking feature requests, I know it's a big ask from an OSS maintainer, but I figured I'd give it a shot.

Thanks!