Fix BaseButton multitouch signals firing for only 1 button

[shiena]

Fixes #118581.

Regression from #110893.

Multitouch button_down / button_up signals only fired for one button at a time. When a second
InputEventScreenTouch press arrived on another button, on_action_event() rejected it because
status.hovering is only set by mouse motion and the event was not treated as an accept event, so
status.press_attempt never became true and no signal was emitted.

Also accept the press when the touch event targets a button whose pressing_inside is already true
(i.e. the touch that started inside the button's rect), so each button processes its own touch stream
independently and multiple buttons can emit button_down / button_up simultaneously.

• Verified on iOS Safari with a Web export of the MRP, pressing 4 buttons simultaneously with 4 fingers.

  multitouch.mp4

• Verified on Android Chrome

  multitouch-android.mp4

[m4gr3d]

Validated that the fix resolves the issue in #116716 where multiple buttons cannot be pressed at the same time.

[Alex2782]

Looks good! Tested on Android and macOS

[Repiteo]

Thanks!