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-uikit/src/view.rs

@@ -571,6 +571,7 @@ impl WinitView {
                             } else {
                                 ButtonSource::Touch { finger_id, force }
                             },
+                            is_macos_activation_click: false,
                         },
                     });
                 },
@@ -629,6 +630,7 @@ impl WinitView {
                                 } else {
                                     ButtonSource::Touch { finger_id, force }
                                 },
+                                is_macos_activation_click: false,
                             },
                         });
                     }