Fixes #4761

This patch adds tests for some code paths we weren't hitting before and removes
some dead code that couldn't be tested because it was unreachable.

In writing a test for #1927, I found a number of bugs in how Draggable shuts
down. Previously it would leak its recongizer. Now it disposes its recognizer
and the recognizer knows how to be disposed cleanly.

Fixes #1927

We have these details objects for the same reason we now have drag details
objects: future extensibility.

Also, make some previously private classes public for better
documentation.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

Wait until the end of the microtask to tell gesture recognizers that
they've won in the gesture arena. This lets recognizers dispose reject
themselves at arbitrary times without triggering gestures in awkward
call stacks.

Fixes #3183

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

There's no reason to make clients supply a PointerRounter and a
GestureArena when constructing gesture recognizers. These objects are
statics and the gesture recognizers can just grab them directly.

Also, remove the callback constructor arguments. Almost no code used
them. Instead, people seem to prefer using the `..` operator to set
callbacks on the recognizers. Removing the arguments removes a bunch of
boilerplate.

This allows us to adjust exactly which gestures we're listening for
during layout, which I'll use to kill a SizeObserver.

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

...even after winning the arena.

This lets it cooperate with other gestures like tap.

The way I implemented this was to refactor the entire Draggable gesture
logic to use a new kind of gesture detector called
MultiDragGestureRecognizer. It works a bit like
MultiTapGestureRecognizer but for drags.

Also some tweaks to the velocity estimator.