Toolkit USE Flags

This page summarizes main points from discussions about toolkit (GTK, QT) USE flags (gtk2, gtk3, qt4, qt5, etc). The centithreads go back to 2005. It seems sometimes the discussion is going in circles, and it's hard to read all of the previous replies. This is an attempt to summarize the main points, pros and cons, to facilitate a more productive and informed effort towards a usable solution.

List of use cases (i.e. scenarios people want to work, or things that happen with packages, etc)
 * Enable just gtk2 for just gtk2 support if available