Threaded mode: the activation policy, before a launcher's runMain launches the app #64

Part of #49. Found while building the renderer's side (sidorares/react-x11#542).

What

initApp({ activationPolicy }) has to run before the app finishes launching, because a Regular launch registers a Dock tile.

Under a launcher, the app's code has not run by then. react-x11's react-x11/cocoa-main, for example, calls initApp() and then runMain() on the main thread before the worker has imported the app's entry.

So an app's own initApp({ activationPolicy: 'accessory' }) from the worker arrives after finishLaunching. This is inferred from the ordering; I have not observed it on screen. The expected result is an agent app that shows a Dock tile until the policy is applied.

Proposed

A way to set the policy before launch that does not need the app's code to have run. For example:

• runMain({ activationPolicy }), which the launcher calls with a value it knows at startup — from an env var, or a flag of its own;
• or initApp reading the policy from an environment variable.

A bundled app already gets this right from LSUIElement in its Info.plist. The shape is the bridge's choice.