ui: services - should not export internal variables (serviceName)

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Currently, stark-ui exports starkAppSidebarServiceName, starkProgressIndicatorServiceName, etc.

Expected behavior

Only the InjectionToken of the services should be exported and not their name.