Web: Only Use PointerEvent for Touch

[JAD3N]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

This builds on top of #2721 by making the mouse handler be the way to handle mouse events instead of pointer. Pointer events are still conditionally used (depending on browser support) for touch events.

By doing this it should resolve issue #2475 as pointer events don't trigger events for mouse buttons pressed while another mouse button is being held. More information can be found here but the main concern is in the first paragraph:

For mouse, it is fired when the device transitions from no buttons pressed to at least one button pressed.

[JAD3N]

Currently I've left all mouse event handlers in mouse_handler.rs but it might be merge that into canvas.rs and add a new handler for touch events as touch_handler.rs and use that as a fallback if pointer events aren't available.

[daxpedda]

This is definitely still a problem even after #2662, I will review this soon.

You don't need to rebase it until I review the approach to this.

[daxpedda]

I replaced this by #2838.

When a pointer button is already pressed, new button presses/releases won't go through pointerdown/pointerup, but through pointermove, [..]

So we shouldn't need to handle both events.