Add CustomOneChildLayout

Make TapTracker and TapGesture private classes

A CustomOneChildLayout is a widget that lets you perform a custom layout in the
simplified setting of sizing and positioning a single child.

Previous these callbacks were leaking the implementation detail that they were
triggered by taps. In a later patch, we're going to add a parameter to
GestureTapCallback that these callbacks won't have.

Related to #1807

We don't want to expose these from the gesture package. This patch moves
DoubleTapGestureRecognizer into tap.dart so we can make TapTracker and
TapGesture private to that file.

Use the presence of handler to determine 'enabled'

Right-aligned text paints offscreen sometimes

Instread of an explicit 'enabled' bool, this uses the presence of the
event handler to determine if a widget is enabled or not. This means
that if you've not passed a handler, your widget will be disabled, which
makes sense, since it wouldn't work anyway.

Adds this feature to checkbox, and ports raised button, flat button, and
radio buttons to this new model.

Adds a checkbox to card_collection that can be disabled.

Hide a (basically bogus) hint from the (soon to be disabled) strong hint
mode in the analyzer that this reveals.

sprite nodes now caches inverse transforms

We were assuming that the text extended from 0.0 to its max content width.
That's not correct for right-aligned text. Instead, we need to layout the text
again at the width we want it to occupy.

Handle mouseover events in pointer event conversion

Fixes #1806

Also:
 - give card_collection an option to turn on or off the edit widgets
 - give card_collection an option to control text alignment (when not editing)
 - give card_collection a "dump" option to aid debugging
 - make the gesture detector report which gestures it is listening to

Restore previous tap behaviors: no timeout, one pointer

Be explicit about whether text styles inherit

Don't use a timeout to cancel tap tracking. Track only one primary pointer
and ignore non-primary pointers. Update tests to reflect desired behaviors.

Fixes #1779, #1780, #1781.

Adds motors to revolute joints and prismatic joints

Now TextStyle has a boolean "inherit" value, which controls whether null values
should use should inherit from the current default text style or whether they
should use their initial values.

Remove an unused variable from input_test

This concludes the fn3 port!

In the stock list, filter out any symbols that have not yet been fetched

Fix Stocks search mode back button.

debugPaintPointersEnabled mode

trivial cleanup in gesture code

better feedback for null child to a Block or Column

Radio<T> and ValueChanged<T>

This changes flutter's updater package to depend on and use the new flx
package. I also did a little cleanup of the Bundle interface.

If you load the example and immediately switch to the Portfolio tab, you may
see exceptions due to symbols that are in the portfolio list but whose
data is not yet present in the stocks map

Make Radio widgets take a type that describes the type of their value,
so that you can catch when you use the wrong value.

Standardise on ValueChanged<Foo> instead of having a FooValueChanged
for every value of Foo.