multi-click support

[ickshonpe]

Objective

Implement minimal multi-click support.
Add double click to select word and triple click to select all support to text input widgets.

Fixes #23874

Solution

New MULTI_CLICK_DURATION constant that sets the max time between clicks for them to count as consecutive.

Added a field count to Click. 1 is a single click, 2 is a double click, 3 is a triple click and so on. It's a u8 and saturates at 255 if you click that many times.

Current multi-click state is stored in a field clicking on PointerButtonState.
The clicking state is cleared after MULTI_CLICK_DURATION without another click.

New SelectWordAtPoint TextEdit.

New observer system on_pointer_click in bevy_ui_widgets::text_input. This queues SelectWordAtPoint for 2 clicks and SelectAll for 3 or more. Click events are dispatched on release, so they happen after Press. The Press for a double click might move the cursor but that doesn't affect the SelectWordAtPoint edit that is dispatched.

The implementation is deliberately minimal, I just added here what was needed for the text input. Left for followups:

• Tracking the initial click position and returning the displacement with Click.
• Spatial tolerance (clicks are only multi-clicks if the pointer hasn't moved too far).
• Configurable multi-click duration.

We could add a separate MultiClick event instead. It seems to me things would be more complicated with both Click and MultiClick observer systems though.

Testing

Text inputs now support double click to select a word and triple click to select all:

cargo run --example multiline_text_input

[alice-i-cecile]

New minigame idea just dropped!

[alice-i-cecile]

Oh lovely. That's very simple!

[SpecificProtagonist]

This should be a resource instead so it can be overwritten.
(I wish that there already was a crate to fetch this setting, but I haven't found one)

[viridia]

There's no crate that does this AFAIK. Instead, we'll eventually need to add OS-specific code in bevy_platform. For example, the windows version looks like this:

#[cfg(target_os = "windows")]
mod platform {
    use super::*;
 
    pub fn get() -> Option<Duration> {
        // GetDoubleClickTime returns the interval in milliseconds.
        // https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getdoubleclicktime
        extern "system" {
            fn GetDoubleClickTime() -> u32;
        }
        let ms = unsafe { GetDoubleClickTime() };
        if ms == 0 {
            None
        } else {
            Some(Duration::from_millis(ms as u64))
        }
    }
}

Also, on Linux there's no single setting, you have to parse the Gnome or KDE settings files, this will take some time at startup.

[SpecificProtagonist]

Which is why a separate crate would be nice – Bevy isn't the only library having this issue.

[ickshonpe]

Should this be a resource instead? (I wish that there already was a crate to fetch this setting, but I haven't found one)

I'm just using a constant here because I wanted to keep this a narrow PR focused on the multi-click implementation. It should definitely be user configurable, or fetched from the OS if possible, though.

[ickshonpe]

There's spatial tolerance as well, which we'd also want to get from the OS. Left it out from this PR because it has to account for scale factor (which is annoying).