Image selection

Removes _selectedImage.
> The resume() function when called in your wrapped _selectSingleNode isn't yet set to the return value of manipulationTestHelper because that function hasn't yet returned. Having a hard time making heads or tails of this.

1. The scope of this call is a function definition, an assignment to `wymeditor._selectSingleNode`. `wymeditor._selectSingleNode` will be called later, triggered by [pre–7 Trident's `mouseup`](https://github.com/wymeditor/wymeditor/blob/bb09bdb209ebb94c3c352f8a857e872631ae0a09/src/wymeditor/editor/trident-pre-7.js#L72).
1. It is ok that `resumeManipulationTestHelper()`, which is not defined (`undefined`) at the time of this function's definition, is included inside, because it is currently set to `undefined`. If you try to call it now, it would be like doing `undefined()` and that will probably yield something like "undefined does not have property `call`" or something like that.
1. Since the call to `wymeditor._selectSingleNode` is inside a `setTimeout` (with zero time) it will run only (and probably immediately) after `resumeManipulationTestHelper` had returned. Thus, in the scope of the anonymous function of this `test`, by that time, when it does run, in this scope `resumeManipulationTestHelper` would have already been the return value of `manipulationTestHelper`.

I hope this is clear enough :)