Feature: Add Save config from GTK frontend

[markmandel]

Just a nice to have feature, especially for those people who aren't as comfortable writing config files.

Once you have a config working in the GTK application, have a "save" or "export" button to save that to the appropriate place locally ($XDG_CONFIG_HOME/lan-mouse/config.toml since I'm on Linux).

Makes it easy to experiment with the gtk frontend first, get everything working, and then persist it for future sessions without having to type it all out.

[feschber]

Yeah I have already started to work on this because the encryption stuff is fairly unusable otherwise.