HttpTileSource: add HttpClient parameter to constructor

[janusw]

Follow-up to #213, #214, #216, #221 ...

The HttpTileSource class should have an optional HttpClient parameter in its constructor.

This parameter should replace the current parameter userAgent and would introduce the possibility to set further header items like a referrer, credentials or an authentication token, so that the methods AddHeader and GetHeaders could be removed.

If no HttpClient parameter is provided, a default HttpClient should be set up that includes no specific header items.

As a side condition, it should not be possible to set a custom HttpClient and a custom tileFetcher at the same time, i.e. there should be two separate constructors, one of which takes an HttpClient parameter, while the other takes a tileFetcher parameter (because using a custom tileFetcher means that the HttpClient will not be used).

[janusw]

If such a breaking change is implemented, one could also take the opportunity to further clean up the HttpClient constructor, as proposed in #216 (comment):

Parameters like 'name', 'persistentCache' and 'attribution' could be removed from the constructor entirely without loss of functionality, because they all can be set via public properties.