Add context menu for linux

This adds a context menu for the linux platform because the right-click event used on macOS isn’t available in Linux, and therefore the context menu is empty and the settings and logout options cannot be used.

Note: There is an issue with node-browser-history (at least for me) if you want to test this on Linux (fixed in !7 (merged)).

Screenshots

Taken from Unity (as it’s the only supported desktop in menubar):

Before	After