Missing Getter and Setter in C# Property

[xuhongxu96]

Per #550, C# has been supported. But the getters and setters cannot be parsed normally now, e.g.:

Property signature not valid, falling back to variable: string Path     = string.Empty

Getters and setters are missing in the declaration signature:

https://github.com/michaeljones/breathe/blob/4ce8c7a0d79f79f0ae3e7c7fbaa9c4fbb5c84888/breathe/renderer/sphinxrenderer.py#L1839-L1855

argsstring is empty. We should consider node.settable and node.gettable to construct { get; } and { set; }