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.
@makhmet Graag alleen Engels hier.

@theuni Well - I initially started from the approach that there was a Style class per network, but that didn't turn out to be needed. The current interface (with a constructor-independent instantiate) leaves some freedom to return to that later if it becomes necessary (ie, if you'd want to paint the background in a network-specific way and offer a method for that). Also the function can return 0 if the network is unknown, unlike a constructor which would have to mess around with exceptions.

Good idea about the RAII though. I'll change the networkStyle in main to a boost::scoped_ptr.