Image selection

Removes _selectedImage.
In pre–7 Trident, the `mouseup` does the [same thing](https://github.com/wymeditor/wymeditor/pull/641/files#r22141769) as the other browsers, with a difference.

The native code that brings up the resize handles is triggered by the `controlselect` event.

The `controlselect` event is triggered later than the `mouseup` event.

This means that if `_selectSingleNode` was called from within the `mouseup` handler, the resize handles would appear synchronously after that call, replacing that text selection with the "control selection".

So we throw the call to select the image on top of the asynchronous calls queue.

This makes reasonably certain that our selection call will run after the native `controlselect` resize handles were drawn.

This results in one of the shortcomings of this PR. That is, in all IEs, the resize handles appear for a flicker before being replaced by the single node (the image) text selection.