This commit effectively moves the definition of these constants
out of the chainparamsbase to their own file.
Using the ChainType enums provides better type safety compared to
passing around strings.
The commit is part of an ongoing effort to decouple the libbitcoinkernel
library from the ArgsManager and other functionality that should not be
part of the kernel library.
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.
