Use QuickList on Linux instead of/in addition to tray icon

[cassidyjames]

Checklist:

• [x] I have checked that there are no issues with similar or the same content

  YOU SHOULD CHECK CLOSED ISSUES ASWELL

• [x] I have checked the FAQ (https://github.com/MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-/wiki/FAQ) and the answer I am looking for is not there
• [x] I have double checked and can reproduce the issue

OS: Linux (elementary OS 5.0 Juno)

GPMDP Version: 4.6.1

Issue Descriptions:

On many Linux distributions (including elementary OS, Pop!_OS, Fedora, and GNOME by default) there is no support for "tray icons" anymore. Instead, many modern Linux desktops support "QuickLists" as context menus on the app icons. Static Quicklists are widely supported and are even a FreeDesktop.org standard via the Desktop Entry Specification's "Actions". Ubuntu, elementary OS, and other OSes also support dynamic QuickLists via the LibUnity LauncherAPI if dynamic content is desired, though I imagine static would be good enough.

[MarshallOfSound]

Happy to accept a PR that does this