Set argument of SetProperty to Variant

[WhyNotHugo]

This function strictly takes a Variant, and other types will fail with error:

No such interface “org.freedesktop.DBus.Properties” on object at path ...

Closes #202

[WhyNotHugo]

I'm not entirely convinced by the results here; it someone passes an int then we're trying to set a property of type int, but if someone passes a Variant<int>, we're also trying to set a property of type int, and not Variant<int>.

I'm not sure which is the best behaviour with the current signature.

[guelfey]

But that would still be compatible to existing code - if you have a property that already is an actual Variant, you need to pass e.g. a Variant<Variant<Int>> here. This just makes this function a bit nicer to use if your property is not a Variant, which I'm fine with.

[WhyNotHugo]

Yeah, the main upside is that it maintains compatibility with existing code, but it's really unexpected behaviour and not a great API.

Maybe it's best to merge this as-is and refine it before a major release (at which point a breaking change might be acceptable).