Add KeyboardModifiers to PointerEvent / Add PointerEvent to clicked closure

[wspl]

I'm creating a List with a multiple selection feature. I need to detect whether the user pressed Shift or Control when clicking, in order to decide which selection operation to perform.

I tried wrapping my window with FocusScope to capture keyboard events. However, it is behaving strangely: It doesn't get focused when I click a list item directly. I need to click the blank area of the window to make it focus, which then allows FocusScope to capture the keys.

I think a better approach to make it work would be to add KeyboardModifiers to PointerEvent. And then expose the PointerEvent struct to the clicked closure. (The pointer-event closure is unstable too, it doesn't triggered in many situaion. Perhaps I will open a new issue to describe this problem.)

[ogoffart]

Yes, we should have a way to detect the keyboard modifier.

This was also discussed in #1280

As a workaround for now, see the example in #2641 (comment)

[ogoffart]

(this was asked for again in #3043 )