macOS: add WindowEvent::PointerButton::is_macos_activation_click

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.

winit-web/src/event_loop/window_target.rs

@@ -296,6 +296,7 @@ impl ActiveEventLoop {
                             state,
                             position,
                             button,
+                            is_macos_activation_click: false,
                         },
                     }]));
                 }
@@ -323,6 +324,7 @@ impl ActiveEventLoop {
                         state: ElementState::Pressed,
                         position,
                         button,
+                        is_macos_activation_click: false,
                     },
                 })));
             }
@@ -351,6 +353,7 @@ impl ActiveEventLoop {
                         state: ElementState::Released,
                         position,
                         button,
+                        is_macos_activation_click: false,
                     },
                 })));
             }