Add link long-press gesture event handling

• Component: UI Events
• Status: RESOLVED WONTFIX

[Peyton Randolph]

Subtask of Add long-press links gesture to Mac
https://bugs.webkit.org/show_bug.cgi?id=134262

Within the link long-press, there are three events we care about: starting the long-press [start], triggering the long-press [trigger], 
and cancelling the long-press before or after triggering [cancel].

The general flow of the event is as follows (the event triggered at a step is placed in square brackets):

- [start] The user mouses down over a link.
  - [trigger] The user keeps the mouse pressed on that link without moving their mouse too much for longer than
              the specified long-press duration.
    - The user releases their mouse press.
    - [cancel] Without releasing their mouse press, the user moves their mouse outside the bounds of the link.
               Doing so cancels the long press, much like pressing a button on OS X and then moving outside 
               that button's bounds cancels the button press.
      - The user releases their mouse press.
      - [trigger] Without releasing their mouse press, the user hovers over the original link again. Loop back
                  to the grandparent's trigger event.
  - [cancel] The user releases their mouse press before triggering the long-press, triggering a normal mouse up.
  - [cancel] The user moves their mouse outside a small region around the original mouse press before triggering
             the long-press.

[Peyton Randolph]

Created attachment 234372 [details]
Event handling

[Peyton Randolph]

Superseded by https://bugs.webkit.org/show_bug.cgi?id=135257