Add `.capture()` on `ElementBuilder`

[TheRedDeveloper]

Summary

Add a .capture() method to ElementBuilder that captures the pointer, preventing it from propagating to parent elements.

Motivation

Currently, when an element is pressed, all parent elements in the hierarchy also receive on_press and on_release callbacks. This might be bad for a button inside a larger clickable area.

Currently, you make such elements .floating(), but then elements can't size around them. While the workaround of checking pointer_over_ids().last() == Some(id) exists, a dedicated .capture() method would be cleaner.

Origin: #55

Proposed API

// Capture the pointer, preventing it from propagating to parent elements
pub fn capture(mut self) -> Self {
    self.pointer_capture_mode = PointerCaptureMode::Capture;
    self
}

Behavior

• on_press and on_release callbacks still fire on elements with .capture() and its children.
• on_press and on_release callbacks don't fire on elements under an element with .capture().
• pointer_over_ids() still includes the element with .capture() and its children.
• pointer_over_ids() doesn't include elements under an element with .capture().
• Floating elements with PointerCaptureMode::Capture continue to capture the pointer.
• .capture().floating(|f| f.passthrough()) still captures the pointer.
• The pointer still propagates to parent elements unless the element has .capture().