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.
Hmm well on the other hand "[testnet]" is already a translation message. So there is no extra work in keeping it. Not sure about regtest though, that's just too obscure.