qt: add network-specific style object

Mainly cleanups: Gets rid of isTestNet everywhere, by keeping track of network-specific theming in a central place.

Also makes GUI no longer dependent on the network ID enumeration, which alleviates concerns about #4802.
I think a NetworkStyle::destroy(networkStyle) or networkStyle->destroy() or so would be a bit more clear here? With just a delete, it's hard to tell where it came from.