We checked for widget types by doing `configuration?.__typename ===
'LineChartConfiguration'` which made the code difficult to read.
In this PR, I introduce type guards for each widget type.
Note: the configuration type is `WidgetConfiguration |
FieldsConfiguration` for now but should be changed to
`WidgetConfiguration` when @Devessier adds FieldsConfiguration to the
backend type `WidgetConfiguration`.
Raphaël Bosi