'change:bounds' event behavior confusing

One would expect `change:bounds` to be fired also when the widget's position has been changed as this leads to a change of the `bounds` property. It is however implemented as an alias of `resize` and is, as documented, only expected to be fired when the widget's size has been changed. Android client's `Resize` event is however based on a native event triggered on bounds change, so it is also fired when the widget's position has changed.

The introduction of a `move` event for differentiation between changes of size and position might help to solve this issue.

See #158 and @8318b22 for more information.