Enhanced Tray icon

[jgresham]

Feature details

The current tray icon is a minimal implementation. Enhancing it with additional functionality describe in comment #520 (comment)

• Colored icons for errors (red), warnings (yellow), notifications (grey)
• Clicking on a node list-item from the tray menu should open the app with that node selected in the sidebar.
• Using the "Quit" menu item should probably prompt the user with an "are you sure?" type of alert informing them of the implications (nodes will stop syncing etc) either by opening the app with the alert displayed in the app or if not possible through an OS system alert? I'm assuming this would shut down all NN related processes until the app gets relaunched.
• Explore if Electron allows a custom Tray menu UI instead of the standard mac menu items which only support a text string

[jgresham]

Notes remaining after merging PR #597

Notes:

currently not a react component but javascript creating html nodes in the custom window; had difficulty and will need more time to figure that out
couldn't get colored notification icon to work
need more time to figure out a better way to handle granular node statuses such as "Initializing", "Syncing", etc so it can be used the same way in Tray, NodePackageScreen, etc.