* Revert "Revert "Redo: Add buttons to gestures (#31819)" (#31912)"

This reverts commit 60a1b2b9.

This reverts commit 18ca3754.

This reverts commit fea2c7d6.

* Revert "Revert "Add buttons to gestures (#30339)" (#31801)"

This reverts commit 8fd7fa49.

* Synthesise kPrimaryButton for unknown devices

* Change TestPointer to a better API

This reverts commit 727e7e82.

* Callbacks of tap, long press, drag, and double tap GR respond to only primary events.
* Add "secondary" callbacks to tap.
* Recognizers only compete on events if there are any related callbacks.
* Add "kSecondaryButton" constant.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

It was 8.0. It's now arbitrarily 18.0.

Changing this required adjusting some tests. Adjusting the tests
required debugging the tests. Debugging the tests required some tools
to help debugging gesture recognizers and gesture arenas, so I added
some. It also required updating some toString() methods which resulted
in some changes to the tree diagnostics logic.

Also I cleaned up some docs while I was at it.