Apps need per-click decisions to follow the macOS convention of accepting
first mouse for low-risk actions (selection, scrolling) but rejecting it
for buttons and destructive actions. Motivated by slint-ui/slint#10451.
Always return `true` from `acceptsFirstMouse:`, and tag the resulting
`PointerButton` events with `is_macos_activation_click: true` (on both
the activating left press and its matching release) so the app can
short-circuit the whole gesture with a single check:
WindowEvent::PointerButton { is_macos_activation_click: true, .. } => return,
Replaces an earlier callback-based design (`accepts_first_mouse` on
`ApplicationHandlerExtMacOS`) that mapped more closely to AppKit but
didn't fit winit's event-driven model and required re-entrancy handling.
Also removes `WindowAttributesMacOS::with_accepts_first_mouse`, which is
now redundant — apps that want to reject activation clicks check the
flag on the per-event instead.
This avoids using JavaScript exceptions to support `EventLoop::run_app`
on the web, which is a huge hack, and doesn't work with the Exception
Handling Proposal for WebAssembly:
https://github.com/WebAssembly/exception-handling
This needs the application handler passed to `run_app` to be `'static`,
but that works better on iOS too anyhow (since you can't accidentally
forget to pass in state that then wouldn't be dropped when terminating).
