PLRPC should function as a background app

[jvyden]

PLRPC should be able to used as a background application. This should increase usage of PLRPC, since not every user will want to go out of their way to start PLRPC each time they'd like rich presence.

• Should be able to run on startup if desired by the user
• No windows should be opened by default, just merely an icon in the system tray
• Clicking on the system tray should bring up the configuration GUI
• Changes in the GUI should save to a json somewhere in AppData or XDG_CONFIG_DIR

Skimming for PRs/issues, I found that Eto.forms should support tray icons for at least Windows and Linux, although I haven't checked for MacOS.

[Zaprit]

macOS shouldn't be an issue as it's currently unsupported in PLRPC

[sudokoko]

macOS shouldn't be an issue as it's currently unsupported in PLRPC

Will probably want to implement macOS support (tracked in #36) before this so that I'm able to test across both platforms.