* Revert "Revert "Add buttons to gestures (#30339)" (#31801)"
This reverts commit 8fd7fa492aaad0d0d4aa062d71b1ecba8b0d4f44.
* Synthesise kPrimaryButton for unknown devices
* Change TestPointer to a better API
* 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.
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.