feat: Allow the passthrough of pointer events inside the highlighted area

[Termtime]

In my use case, I require the user to click some buttons inside the highlighted area, afterwards I wanted to continue the showcase, so what I did is:

• Added PassthroughMode enum that declares the behavior of the passthrough, by default RelativeBubbleSlide uses the PassthroughMode.NONE option which absorbs all pointer events and advances the showcase, as it currently is doing.
• When PassthroughMode.INSIDE_WITH_NOTIFICATION is used, several things happen:
  • A CustomClipper is used instead of a CustomPainter, which clips around the highlighted area, leaving the highlighted area exposed to pointer events
  • The default behavior of continuing the showcase when the user clicks anywhere is removed. To continue the showcase, the user MUST dispatch a BubbleShowcaseNotification Notification that lets the BubbleShowcase object know that it should continue.
    • This is thought so the user can easily do async tasks and then notifiy the BubbleShowcase when it should continue.
    • Pretty useful as well if you want to wrap the BubbleSlide with a GestureRecognizer to override the default behavior of the button or element you want the user to interact with.
• Added the option of "padding" (Default of 0, like it currently is) to the highlighted area. The amount of padding is added to the Position that is gotten from the highlighted widget.
• Updated the example with a 4th slide which includes highlightPadding and PassthroughMode.INSIDE_WITH_NOTIFICATION
• Made a comment regarding the initialDelay property be a doc comment instead of being a normal comment.

Default current behavior is retained while adding the new options and functionality.

[Termtime]

@Skyost Check it out when you can and LMK if you require anything else.

[Skyost]

Will check this week 👍