Modify startup on Windows and macOS so event loop is running when startup is called

[freakboy3742]

Modifies the startup sequence of Windows and macOS apps so that the event loop is guaranteed to be running at the point startup is invoked. This ensures it is possible to call asyncio.create_task().

This isn't an issue on other platforms:

• On GTK, startup is triggered by the GTK app's activate message, which requires the app loop is running
• On iOS, startup is triggered by the receipt of the application:didFinishLaunchingWithOptions: selector on the AppDelegate, which requires the app loop is running
• On Android, startup is triggered explicitly when the main loop is started; but the main loop is cooperative, based on the Java context handling events.
• On Web, there's no explicit event loop creation; asyncio works as soon as the interpreter has started.
• On Textual, the app isn't created until the app itself is mounted.

Fixes #2834.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

My initial concern was this is going to change the order of events at startup...but I think in reality it just slightly delays when the last bit of startup code runs 👍🏼