Fix + enhancement needed for Slider

This PR is for review.
- fixes https://github.com/ibm-js/delite/issues/161 FormValueWidget: wrong pendingOnChange on certain cases
- Introduces an enhancement of _handleOnChange that is needed by deliteful/Slider (see PR https://github.com/ibm-js/deliteful/pull/131)

**Content**
- a property `intermediateChange (true|false)` is added to the change event to indicate if the event is the result of an intermediate change (user is moving the Slider handle).
- a new parameter `force` is introduced to `_pendingOnChange`:

When `intermediateChanges` is true, setting `force` to true allows to force call to onChange() even
if the value hasn't change since the previous call. Useful for Slider to ensure a change event
is fired when pointer or key is released. Force the property `event.intermediateChange` tofalse to
allow application to differentiate intermediate changes from the last change that is the actual
user selection. Defaulted to false if not specified.

**Other changes**
- renamed `priorityChange` > `fireOnChange`
- `_pendingOnChange` documentation