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.
Move checkpoint data selection to chainparams
Add fTestnetToBeDeprecatedFieldRPC to CChainParams
Remove CChainParams::NetworkID()