Rename OdfUtils.isEmpty to OdfUtils.hasNoODFContent,
which is much more precise.
Additionally, it should return true on textNodes only if
they are children of ODF elements.
Add property mergedParagraphStyleName to OpRemoveText spec.

* Make TextController specify the mergedParagraphStyleName
when generating a RemoveText operation. No longer let
OpRemoveText decide the merge order from the context.

* Remove the text:style-name attribute on a paragraph
if the aforementioned property is an empty string.

* Add tests for this.