enh: Add a .frame() method to Displays

[glemaitre]

Right now, our displays have a single method .plot (and potentially .set_style). I most cases, it would be great to expose a .frame property returning a dataframe containing all the information allowing to make the plot.

For instance, for the feature importance, it could be a dataframe with the name of the features as row and the importance as column. For the ROC curve, it could be the fpr, tpr, and the associated thresholds.

[glemaitre]

Actually, I think that we could make it a method such that we can offer the possibility to have name easy to index vs. name to make the rendering nice, in the same spirit of the flat_index.