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.
Does it matter where it came from? In principle any wrap-up code could be done in the destructor.
I'm fine with adding a NetworkStyle::destroy(obj) function if you think it's more symmetric, but unlike instantiate there is no reason to do anything special based on the network, it would just call delete anyway. I don't like the ->destroy() style as it involves `delete self`.