Add heat map visualisation (via new or existing widget) to story navigator

[ThijsVroegh]

Desire to visually represent the story in terms of a heatmap: upper-left is the begin, down-right the end of the story. Each individual block of the heatmap represents a sentence. Next, these heatmap blocks should be colored based on word occurences or match with the dominant sentiment of the particular sentence. For example, this allows to visually see whether the first part of a story has a lot of 'zij' usages, whereas in the latter part 'hij' is dominant