Chart Mappings & Data Labels

[jonmuell]

You can now manipulate the data labels in the chart widgets (e.g. to localize the date format) and provide a custom chart mapping in JSON format (useful for translation).

[jonmuell]

@chaaya @damoonrashidi @chlirre

With this PR it is also possible to change the date format as in #4, however, I solved this differently: In #4 you specify the pattern and can use a parse function in chartMappings.ts. I'd like to keep chartMappings.ts clean as I need to provide it as JSON from our Angular app. Therefore I added a DataLabelTransformer which does basically the same but if you want to use it you have to do the whole logic of parsing and transforming the date yourself. The way #4 solves this might be simpler but the problem is that call_length_distribution has a data label missed which we need to translate. To access it we can use the DataLabelTransformer aswell, so my opinion is to stick with this one as it can solve both problems.

What do you think?