always require the icon library

the only exception is Android, there we never did setup Breeze in any way and this is marked as porting aid

consolidate the fallback theme setting

this will ensure that Breeze is always at least the fallback

if the app needs more it can opt-in like before with the

KIconTheme::initTheme helper

remove the old code we needed for craft on Windows and macOS in the KF5 time