OT improvements for undo in pasting scenarios

[scofalik]

I discovered an error concerning undo in a scenario with paste and delete.

This test doesn't pass:

john.setData( '<paragraph>Ab[]cd</paragraph><paragraph>Wxyz</paragraph>' );

john.editor.model.insertContent(
	new DocumentFragment( [
		new Element( 'paragraph', null, new Text( 'Foo' ) ),
		new Element( 'paragraph', null, new Text( 'Bar' ) )
	] )
);

john.setSelection( [ 1, 3 ], [ 2, 2 ] );

john._processExecute( 'delete' );

expectClients( '<paragraph>AbFoo</paragraph><paragraph>Baryz</paragraph>' );

john.undo();

expectClients( '<paragraph>AbFoo</paragraph><paragraph>Barcd</paragraph><paragraph>Wxyz</paragraph>' );

john.undo();

expectClients( '<paragraph>Abcd</paragraph><paragraph>Wxyz</paragraph>' );

There are two problems, with split x split transformation and merge x split transformation. These are some edge cases that weren't yet covered in OT.