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.
It doesn't really matter, it was just a suggestion for readability.

Out of curiosity though, why not just create a normal scoped object in main and pass around a pointer to it? I suspect I'm missing something obvious.