gpui: Allow the application to run after all windows are closed

[Guibi1]

This PR adds the option to prevent the event loop from exiting when the last window is closed by the user.

This is needs more thought because:

1. The naming might not be the best
2. This wasnt tested on all platforms
3. MacOS is special and we need to make a decision on the api about the fact that it has the opposite behaviours as the other os

Release Notes:

• Added the option to keep the application running with no window open

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[Guibi1]

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[JunkuiZhang]

I’m curious why do you need this feature? AFAIK, on Windows pretty much all apps exit once the window is closed, unless they’ve got a tray icon like Steam.

[Guibi1]

Exactly. Currently its impossible to do a system tray without running another event loop outside of gpui and doing an Application:new each time you need to open the app.

[unsecretised]

I found a way to add a tray icon if thats what you are looking for...
https://github.com/TeenCoder159/macos-template/blob/main/gpui/src/main.rs

however, exposing the event loop + adding a bunch more configurable options is needed (such as setting window visibility)

[mikayla-maki]

Hello!

I think a better fix would be to remove the behavior from the GPUI platform entirely, so that all applications stay running after the last window is closed.

Then we can re-implement this at the application level.

[Ciantic]

I came here from search. For completness, here is example how to turn the feature on.

This is not released in crates (version 0.2.2) yet, but it works like this in the branch.

Application::new()
  .with_quit_mode(QuitMode::Explicit)

or

Application::new().run(|cx: &mut App| {
  cx.set_quit_mode(QuitMode::Explicit);