Allow checking if a destination of a specific type has already been added

Comments from users:

The initial problem which prompted this addition was that I wanted to have both QsLog in my library (.dll) and in my calling instance (.exe). This change exposes the list of destinations so that you can determine if a destination is already added or not.
Hi guys, I have a case, where a cross-platform shared object (SO / DLL) uses QsLogging. As this is an independent deliverable which can be linked to a variety of different applications, there is a great chance that one or more of the main applications are using QsLogging as well. In this case, the problematic behaviour described by Niklas appears.

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