Define your own stakeholders in a LES

[DorinevanderVlies]

Currently ETM-SA works with default stakeholders.
For customers AC1, AC1a, AC1b, AC2, etc... can be selected.

To make the LES more readable to external viewers it would be nice if I can define my own stakeholders or a LES.
For example: Flats, small houses, shops, etc

Just custom made customer labels would be nice, but I can imagine that this feature would be desired for other stakeholders as well. For example if we somehow want to implement a third party to pay for isolation investments.

notifying @AlexanderWirtz

[ChaelKruip]

Related to #1204