Tray Icon integration

[nate-trojian]

Adds a basic integration with tray_icon. Usage has been put behind the tray-icon feature flag.

Added the ability to:

• Define a Tray Icon using either text or image
• Process events for:
  • Mouse entering the icon
  • Exiting the icon
  • Pressing the mouse over the icon
  • Releasing the mouse over the icon
  • Double clicking the icon
  • Clicking a menu item
• Add a menu to the Icon
• Add a menu item with just text, a checkbox, an icon, or a predefined menu item

Current limitations:

• No ability to modify the tray icon during the running of the application
• Tray icon events must be processed from a subscription due to the fact they aren't tied to a window
• Menu has to be open in order for "accelerators" to be used (limitation of how menus work in tray icon)
• Clicking a predefined menu item does not generate a menu clicked event (this is a limitation inside of muda)

Screenshot of running example:

[dtzxporter]

I'd like to see more of the code be disabled completely when the feature is disabled, IMO, nothing related to tray icon, even stubs, should be included because we have #[cfgs].

[n1ght-hunter]

I'd like to see more of the code be disabled completely when the feature is disabled, IMO, nothing related to tray icon, even stubs, should be included because we have #[cfgs].

agreed that was just the first thing that popped out to me. really i think there should be pretty much no code included without enabling the feature

[nate-trojian]

I'd like to see more of the code be disabled completely when the feature is disabled, IMO, nothing related to tray icon, even stubs, should be included because we have #[cfgs].

Because of how tightly integrated this feature needs to be within the system, I didn't originally want to do this. I think it hurts the readability of the code and will make it harder to add functionality to this feature in the future.

However, I've made the change to move the guards to all the public interfaces.

[Decodetalkers]

https://github.com/tauri-apps/tray-icon/blob/dev/examples/winit.rs#L117-L125

I think on linux it needs gtk to show the tray icon