Remove "paper" as default reference when adding annotations/shapes/images

[jonmmease]

This PR updates add_annotations/add_shapes/add_images so that if no row/col are specified, the xref/yref properties are left unset rather than being set to "paper". Now, to use add_annotations to add an annotation in "paper" coordinates you leave row/col unset and simply pass xref="paper" and yref="paper kwargs.

Similarly, select_annotations no longer accepts row="paper"/col="paper. Instead, to select only the annotations in paper coordinates, you would use xref="paper", yref="paper" in the selector.

@nicolaskruchten

Closes #1881

[nicolaskruchten]

@emmanuelle this resolves this issues you raised right? 💃 from me if so!

[emmanuelle]

Yes, looks good ! 💃 I might have to update the doc tutorial.