DO NOT MERGE! Vice 4 Changes

[dqh-au]

This pullreq exists to provide a place to inspect changes that have been made in the vice-4 branch.

High level changes (all work is in-progress):

• Headless builds include a TCP UI server, allowing external processes to connect and receive screen images. The UI server is intended for localhost use only. TCP has been used as a portable IPC mechanism - in time it could be replaced with lighter weight OS specific IPC mechanisms.
• A UI client lib is provided to allow development of new VICE UI clients
• A headless build can automatically launch a UI binary via the -ui command line option or corresponding settings resource. The launched UI client is passed the local TCP port on the command line.
• GTK3 and SDL2 builds no longer produce screen images (inc CRT filter) on the emulation thread. GTK3 builds shift this work to dedicated render threads (one per window) and SDL2 builds shift this work to the UI thread.
• SDL1 support has been removed.

[0wwafa]

@dqh-au latest GTK3 win64 version "works" but crashes as I open the about menu