Add ability to toggle tray icon on Linux

[tonyjames]

On macOS there is the ability to hide the tray icon and only be notified via the dock icon. It would be useful to have the ability to hide the tray icon on Linux as well and rely on desktop notifications.

[xTeixeira]

+1

Very useful for Linux since GNOME has removed tray icon support.

[dzero1]

Yet not working on Linux mint 17.3 64bit. (Yes, I tried with toggling the Tray icon menu item for more than 10 times.)

[tassoevan]

@dzero1 Is libappindicator1 or libappindicator3-1 installed? Also, which desktop environment are you using?