This patch fixes a collection of issues with widgets involved in text
editing:

  * Fire widget.onChanged on EditableText value change:
    The value of an EditableText is composed of the text value as well
    as other editing-related data such as selection-related information.

    Previously, widget.onChanged() was only called for updates via
    updateEditingValue(). For pastes via a TextSelectionOverlay, updates
    are signalled via _handleSelectionOverlayChanged(), which only ever
    triggered widget.onSelectionChanged(), but not widget.onChanged().

    Both updateEditingValue() and _handleSelectionOverlayChanged()
    perform the value update via _formatAndSetValue(), which is where
    this patch moves the widget.onChanged() call.

  * Correctly update TextFormField value on edits via controller:
    The textual value of a TextFormField exists in two locations:
      1. FormField.value, as with all FormFields and subclasses.
      2. TextEditingController.value associated with the TextField
         underlying the TextFormField.

    Previously, edits to the TextEditingController associated with a
    TextFormField resulted in updates to the rendered TextField widget,
    but did not update TextFormField.value. FormField.value is updated
    via FormField's onChanged function, which is called from the
    EditableText underlying the TextField underlying the TextFormField.
    EditableText only fires onChanged when it receives changes from the
    engine. It does not fire onChanged for changes made to the
    underlying TextController, since the owner of the TextController is
    the one making these changes and thus, already aware of them.
    FormField, however, *does* need to listen to these changes to update
    its value.

  * Adds an initialValue parameter to the TextFormField constructor:
    FormField's constructor already takes an initialValue parameter,
    which specifies the initial value in the field, which is also the
    value to which reset() returns the field.

    Previously, TextFormField took its initial value from the controller
    value (if a controller was passed in) or the empty string (if not).
    This had the undesirable effect that calling reset() always resets
    the value to the current value of the controller... i.e., does
    nothing.

    We now take an initial value explicitly.