Use MouseEvent.buttons for button state tracking

[CendioHalim]

Instead of keeping track of button states ourselves by looking at MouseEvent.button, we can use the MouseEvent.buttons which already contains the state of all buttons.

Note that in MouseEvent.button, button numbers are mapped as follows:

0 -> Left
1 -> Middle
2 -> Right
3 -> Back
4 -> Forward

But when looking at the MouseEvent.buttons property, the bits middle and right are switched, which is confusing:

0 -> Left
1 -> Right
2 -> Middle
3 -> Back
4 -> Forward

With this change, #1919 could be modified to work with Safari as well.

Unfortunately, the internal _mouseButtonStateMask state is not removed in this PR as it would require a large rewrite of the general architecture.

Tested on:

• Linux
  • Firefox
  • Chrome
  • Epiphany
• Windows 11
  • Edge
  • Chrome
  • Firefox
• macOS
  • Safari
  • Edge
  • Chrome
  • Firefox
• iPad
  • Safari