Introduce a PlatformStyle to handle platform-specific customization of
the UI.
This replaces 'scicon', as well as #ifdefs to determine whether to place
icons on buttons.
The selected PlatformStyle defaults to the platform that the application
was compiled on, but can be overridden from the command line with
`-uiplatform=<x>`.
Also fixes the warning from #6328.
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.
