feat: tray icon to allow NN to run in background

[jgresham]

• Show each node and its status in the dropdown
• Add open app option
• better icon from @ddannehh
• Show error icon if a user started node crashed (was not stopped by user) (handle system restart cases) @ddannehh

Icon specs:

Purpose

This allows NN to run in the background by giving users a "hard quit" option under the tray icon (similar to how 1Password works). The purpose is to keep a tray icon updated, monitor node's statuses and alert the user when a node is down, and to continuously measure node usage for charts in-app.
It also will allow a user to know if a node has crashed (error icon in tray) instead of having to open NiceNode window

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[ddannehh]

Icon

Icon below should be according to the MacOS requirements.

In terms of reflecting a status I'm not sure what is possible within the Electron environment.

Guidelines say it has to be monochrome, but there are plenty of apps that add color or even dynamic numbers to the tray icon, see example below.

I've added a monochrome version with bullet to the png files for now.
DOWNLOAD HERE: NNTrayIcons.zip
(Contains both default and @2x versions)

Menu

Basic proposal

This would be a "basic" implementation close to the default MacOS menu's but I guess we'd benefit from a more custom design where we we could add a bit of color or display additional data like whether the node is running on mainnet/testnet etc.
Again, not sure how far we can take this within Electron.

UX

Some things that came to mind:

• 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.

[jgresham]

@ddannehh I merged a working version for now, but it is missing out on the full design & alert icon thinking. I've tried to copy or summarize the yet to do features in #526.