Explain predictions - enable longer labels

[PrimozGodec]

The current limit for label lengths in Explain Prediction widget can be set to a maximum of 500 characters. In cases when feature names are long it is not sufficient. I and @lanzagar propose the following two solutions:

1. Increase the limit to 800
2. Implement the mechanism that on hover labels extend over the graph or same happen on the press of special keyboard key.