QIcon theme revamp

The QIcon theme machinery/APIs/docs could use some revamp. The original design was based on the freedesktop icon spec, but we should not limit ourselves to that. For example, macOS 11+ now has a comprehensive icon set known as SF Symbols https://developer.apple.com/sf-symbols/

Some steps/ideas:

• Let the string based QIcon::fromTheme look up the strings directly in the underlying theme. On freedesktop platforms that will be 1:1 with what we have today. On macOS the names from SF Symbols will be mapped.
  • This behavior change should be fine, as no non-freesktop platform implements this today anyways
• Add a QIcon::StandardIcon enum and fromTheme overload that matches the existing freedesktop spec, mapping to the same names on freedestkop platforms, and to the relevant names on SF Symbols.
• Guard the QIconCacheGtkReader behind a GTK configure ifdef, or possibly move to the Gtk3 platform theme if possible